v1.0.3

add multi lang
2026-07-11 20:03:15 +02:00
parent f0813894a8
commit ae3b435b51
116 changed files with 1543 additions and 484 deletions
--- a/.env.example
+++ b/.env.example
@@ -1,20 +1,20 @@
-# Skopiuj ten plik do .env i uzupełnij wartości: cp .env.example .env
+# Copy this file to .env and fill in the values: cp .env.example .env

-# --- Bezpieczeństwo ---
-# Wygeneruj losowy sekret np. poleceniem: openssl rand -hex 32
+# --- Security ---
+# Generate a random secret with, e.g.: openssl rand -hex 32
 JWT_SECRET=change-me-to-a-random-secret

-# --- Adres aplikacji ---
-FRONTEND_URL=https://twoja-domena.pl
+# --- App URL ---
+FRONTEND_URL=https://example.com

-# --- Domena (routing Traefik) ---
-DOMAIN=twoja-domena.pl
+# --- Domain (Traefik routing) ---
+DOMAIN=example.com

-# --- Sieć zewnętrzna Traefik ---
+# --- External Traefik network ---
 TRAEFIK_NETWORK=traefik_public

-# --- SMTP (powiadomienia mailowe, reset hasła) ---
-# Pozostaw SMTP_HOST puste, aby wyłączyć wysyłkę e-maili.
+# --- SMTP (email notifications, password reset) ---
+# Leave SMTP_HOST empty to disable sending emails.
 SMTP_HOST=
 SMTP_PORT=587
 SMTP_SECURE=false
--- a/21
+++ b/21
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Kacper
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
--- a/README.md
+++ b/README.md
@@ -1,149 +1,165 @@
-# KtoCo — wspólne wydatki dla grup, par i współlokatorów
+# WhoWhat — shared expenses for groups, couples and roommates

-Aplikacja webowa (PWA) do dzielenia się wydatkami w gospodarstwie domowym — kto ile wydał, kto komu jest winien, z historią, statystykami i rozliczeniami. Obsługuje dowolną liczbę osób w gospodarstwie i dowolną liczbę gospodarstw na użytkownika. Mobile-first, instalowalna na telefonie, działa częściowo offline.
+A web app (PWA) for splitting expenses within a household — who paid what, who owes whom, with history, stats and settlements. Supports any number of people per household and any number of households per user. Mobile-first, installable on your phone, works partially offline. Available in Polish and English, with more languages easy to add.

-## Funkcje
+## Features

- **Dashboard** — kafelki „kto komu ile jest winien" (automatycznie uproszczone do minimalnej liczby przelewów dla całej grupy) z przyciskiem „Rozlicz się", wykres kołowy wydatków wg kategorii, podsumowanie miesiąca.
- **Dodawanie wydatku** — duże pole kwoty, kategorie z ikonami, wybór płacącego, trzy tryby podziału: po równo (między wszystkich członków), dokładny podział, całość na jedną osobę.
- **Historia** — lista wydatków z filtrami (miesiąc/kategoria/płacący), edycja i usuwanie.
- **Statystyki** — wydatki miesiąc do miesiąca, porównanie konsumpcji członków gospodarstwa, ranking kategorii.
- **Wiele gospodarstw** — użytkownik może należeć do wielu gospodarstw jednocześnie i przełączać się między nimi (Ustawienia → „Twoje gospodarstwa"); każde gospodarstwo może mieć dowolną liczbę członków, dołączanych tym samym kodem zaproszenia.
- **Ustawienia** — przełącznik i zarządzanie gospodarstwami (zmiana nazwy/waluty, usuwanie członka, usunięcie całego gospodarstwa, zaproszenia), własne kategorie (z edycją nazwy/ikony), motyw jasny/ciemny/systemowy, powiadomienia mailowe, eksport CSV.
- **Konto** — rejestracja e-mail/hasło, logowanie, zmiana nazwy, zmiana hasła, przypomnienie/reset hasła mailem, usunięcie konta.
- **PWA / offline** — instalowalna na ekranie głównym telefonu, cache widoków, wydatki dodane offline trafiają do kolejki i synchronizują się automatycznie po powrocie sieci.
+- **Dashboard** — "who owes whom" tiles (automatically simplified to the minimum number of transfers for the whole group) with a "Settle up" button, a pie chart of expenses by category, and a month summary.
+- **Add expense** — large amount field, categories with icons, payer selection, three split modes: equally (among all members), exact amounts, or the full amount to one person.
+- **History** — expense list with filters (month/category/payer), edit and delete.
+- **Stats** — month-over-month expenses, comparison of household members' spending, category ranking.
+- **Multiple households** — a user can belong to several households at once and switch between them (Settings → "Your households"); each household can have any number of members, who join with the same invite code.
+- **Settings** — switch and manage households (rename/change currency, remove a member, delete a household, invites), custom categories (rename/re-icon), light/dark/system theme, **language switcher (Polish/English)**, email notifications, CSV export.
+- **Account** — email/password registration, login, rename, change password, password reminder/reset by email, account deletion.
+- **PWA / offline** — installable on your phone's home screen, cached views, expenses added offline are queued and sync automatically once the connection is back.

-## Stack technologiczny
+## Tech stack

-| Warstwa | Technologia |
+| Layer | Technology |
 |---|---|
 | Frontend | React (Vite), react-router-dom, TanStack Query, Recharts, vite-plugin-pwa, Material Symbols (Google Fonts) |
 | Backend | Node.js + Express, better-sqlite3, JWT (jsonwebtoken), bcryptjs, nodemailer |
-| Baza danych | SQLite (plik na bind mouncie `./sqlite`) |
-| Infrastruktura | Docker Compose, nginx (serwuje frontend + proxy `/api`), opcjonalnie Traefik (TLS + routing domenowy) |
+| Database | SQLite (a file on a bind mount at `./sqlite`) |
+| Infrastructure | Docker Compose, nginx (serves the frontend + proxies `/api`), optionally Traefik (TLS + domain routing) |

-## Struktura projektu
+## Internationalization (i18n)
+
+The UI ships in Polish and English out of the box, switchable at any time from Settings → Language (persisted to `localStorage`, and to the user's account once logged in, so transactional emails and CSV exports match their preference too). Backend API errors are returned as stable machine-readable codes (e.g. `invalid_credentials`) and translated client-side — the server never hardcodes user-facing language.
+
+Frontend translations live in `frontend/src/i18n/locales/<lang>/<namespace>.json`, one JSON file per component/page, auto-discovered at build time (no registration step). **To add a new language:** copy the `locales/en/` directory to `locales/<code>/`, translate every value, and add `{ code, label }` to `frontend/src/i18n/languages.js`. Backend-rendered content (emails, default category names, CSV headers) lives in `backend/src/i18n.js` — add the new language code there too if you want those translated as well.
+
+## Project structure

 ```
-ktoco/
-├── docker-compose.yaml       # jedyny plik potrzebny do uruchomienia całości
-├── .env                      # konfiguracja (sekrety, domena, SMTP) — NIE commitować
-├── .env.example              # szablon konfiguracji do skopiowania
-├── sqlite/                   # bind mount — tu leży plik app.db (trwałość danych)
+whowhat/
+├── docker-compose.yaml       # the only file needed to run everything
+├── .env                      # configuration (secrets, domain, SMTP) — do NOT commit
+├── .env.example              # configuration template to copy
+├── sqlite/                   # bind mount — app.db lives here (data persistence)
 ├── backend/
 │   ├── Dockerfile
 │   └── src/
-│       ├── index.js              # Express app, montowanie routerów
-│       ├── db/                   # schema.sql + połączenie better-sqlite3
-│       ├── middleware/auth.js     # weryfikacja JWT
+│       ├── index.js              # Express app, router mounting
+│       ├── i18n.js                # server-rendered translations (emails, CSV, default categories)
+│       ├── db/                   # schema.sql + better-sqlite3 connection
+│       ├── middleware/auth.js     # JWT verification
 │       ├── routes/                # auth, households, categories, expenses, settlements, stats
-│       └── utils/                 # obliczanie salda, mailer (nodemailer), pomocnicze household
+│       └── utils/                 # balance calculation, mailer (nodemailer), household helpers
 └── frontend/
    ├── Dockerfile                 # multi-stage: build (node) -> serve (nginx)
-    ├── nginx.conf                 # proxy /api -> backend:3000
-    ├── vite.config.js             # konfiguracja PWA (manifest, service worker)
+    ├── nginx.conf                 # proxies /api -> backend:3000
+    ├── vite.config.js             # PWA config (manifest, service worker)
    └── src/
+        ├── i18n/                  # I18nContext, per-namespace locale JSON files
        ├── pages/                 # Dashboard, AddExpense, History, Stats, Settings, Login, Register, ...
-        ├── components/            # BottomNav, wykresy, formularze, Icon, Switch, ...
-        ├── api/                   # klient fetch + hooki React Query
-        ├── auth/                  # kontekst autoryzacji (JWT w localStorage)
-        ├── household/             # kontekst aktywnego gospodarstwa (lista + przełączanie)
-        ├── theme/                 # kontekst motywu jasny/ciemny/systemowy
-        └── offline/                # kolejka IndexedDB + synchronizacja po powrocie sieci
+        ├── components/            # BottomNav, charts, forms, Icon, Switch, LanguageSwitcher, ...
+        ├── api/                   # fetch client + React Query hooks
+        ├── auth/                  # auth context (JWT in localStorage)
+        ├── household/             # active household context (list + switching)
+        ├── theme/                 # light/dark/system theme context
+        └── offline/                # IndexedDB queue + sync on reconnect
 ```

-## Uruchomienie
+## Running it

-Wymagany jest tylko Docker (z pluginem Compose).
+Docker (with the Compose plugin) is the only requirement.

 ```bash
 cp .env.example .env
-# uzupełnij .env (patrz sekcja niżej) — nie trzeba edytować docker-compose.yaml
+# fill in .env (see below) — no need to edit docker-compose.yaml
 sudo docker compose up -d --build
 ```

-Aplikacja będzie dostępna pod `http://localhost:8856` (oraz pod domeną z Traefika, jeśli skonfigurowana — patrz niżej).
+The app will be available at `http://localhost:8856` (and at your Traefik domain, if configured — see below).

-## Konfiguracja (`.env`)
+## Configuration (`.env`)

-Cała konfiguracja wdrożeniowa znajduje się w `.env` — `docker-compose.yaml` nie wymaga edycji.
+All deployment configuration lives in `.env` — `docker-compose.yaml` doesn't need editing.

-| Zmienna | Opis | Domyślnie |
+| Variable | Description | Default |
 |---|---|---|
-| `JWT_SECRET` | Sekret do podpisywania tokenów logowania. Wygeneruj: `openssl rand -hex 32` | — (wymagany) |
-| `FRONTEND_URL` | Publiczny adres aplikacji, używany w linkach w mailach (np. reset hasła) | `https://ktoco.kzbikowski.pl` |
-| `DOMAIN` | Domena, pod którą Traefik wystawia aplikację | `ktoco.kzbikowski.pl` |
-| `TRAEFIK_NETWORK` | Nazwa istniejącej zewnętrznej sieci Docker, do której podłączony jest Traefik | `traefik_public` |
-| `SMTP_HOST` | Adres serwera SMTP. Puste = wysyłka maili wyłączona (tylko log w konsoli) | — (opcjonalne) |
-| `SMTP_PORT` | Port SMTP | `587` |
-| `SMTP_SECURE` | `true` dla połączenia SSL/TLS od razu (port 465), inaczej `false` (STARTTLS) | `false` |
-| `SMTP_USER` / `SMTP_PASS` | Dane logowania do SMTP | — |
-| `SMTP_FROM` | Adres nadawcy w wysyłanych mailach | `SMTP_USER` |
+| `JWT_SECRET` | Secret used to sign login tokens. Generate with: `openssl rand -hex 32` | — (required) |
+| `FRONTEND_URL` | Public URL of the app, used in email links (e.g. password reset) | `https://example.com` |
+| `DOMAIN` | Domain Traefik should expose the app on | `example.com` |
+| `TRAEFIK_NETWORK` | Name of the existing external Docker network Traefik is attached to | `traefik_public` |
+| `SMTP_HOST` | SMTP server address. Empty = email sending disabled (console log only) | — (optional) |
+| `SMTP_PORT` | SMTP port | `587` |
+| `SMTP_SECURE` | `true` for immediate SSL/TLS (port 465), otherwise `false` (STARTTLS) | `false` |
+| `SMTP_USER` / `SMTP_PASS` | SMTP login credentials | — |
+| `SMTP_FROM` | Sender address on outgoing emails | `SMTP_USER` |

-Bez skonfigurowanego SMTP aplikacja działa normalnie — funkcje „reset hasła" i „powiadomienia mailowe" po prostu nie wysyłają realnych maili (backend loguje w konsoli, że wysyłkę pominięto).
+Without SMTP configured the app works normally — "password reset" and "email notifications" simply don't send real emails (the backend logs to the console that sending was skipped).

-## Dane / trwałość
+## Data / persistence

-Baza SQLite leży w `./sqlite/app.db` na hoście (bind mount, nie nazwany wolumen Dockera) — łatwo ją skopiować, zbackupować albo podejrzeć narzędziem `sqlite3` bez wchodzenia do kontenera.
+The SQLite database lives at `./sqlite/app.db` on the host (a bind mount, not a named Docker volume) — easy to copy, back up, or inspect with the `sqlite3` CLI without entering the container.

-## Wdrożenie za Traefikiem
+## Deploying behind Traefik

-Serwis `frontend` jest podłączony do zewnętrznej sieci `traefik_public` (nazwa konfigurowalna przez `TRAEFIK_NETWORK`) i ma etykiety Traefika (routing po domenie z `.env`, TLS przez `tls-resolver`). Warunek: sieć `traefik_public` musi już istnieć na hoście (tworzy ją zwykle stack samego Traefika):
+The `frontend` service is attached to the external `traefik_public` network (name configurable via `TRAEFIK_NETWORK`) and carries Traefik labels (domain routing from `.env`, TLS via `tls-resolver`). Prerequisite: the `traefik_public` network must already exist on the host (normally created by the Traefik stack itself):

 ```bash
-docker network create traefik_public   # tylko jeśli jeszcze nie istnieje
+docker network create traefik_public   # only if it doesn't exist yet
 ```

-Port `8856` frontend jest dodatkowo opublikowany bezpośrednio na hosta — przydatne przy testach lokalnych równolegle z dostępem przez Traefik.
+The frontend's port `8856` is also published directly on the host — useful for local testing alongside access through Traefik.

-## Model danych (SQLite)
+## Data model (SQLite)

- `users` — konta (e-mail, hash hasła, preferencja powiadomień mailowych)
- `households` — gospodarstwa domowe (nazwa, waluta)
- `household_members` — członkowie gospodarstwa (dowolna liczba osób; użytkownik może być w wielu gospodarstwach naraz)
- `invites` — kody zaproszeń do gospodarstwa (ważne 7 dni, wielokrotnego użytku — nie wygasają po jednym dołączeniu)
- `password_resets` — jednorazowe tokeny resetu hasła (ważne 1h)
- `categories` — kategorie wydatków (nazwa, ikona Material Symbols, kolor)
- `expenses` — wydatki (kwota, płacący, kategoria, data, typ podziału)
- `expense_shares` — finalny podział wydatku między członków gospodarstwa (niezależnie od typu podziału zawsze sumuje się do kwoty wydatku)
- `settlements` — historia rozliczeń („Rozlicz się")
+- `users` — accounts (email, password hash, email notification preference, UI language)
+- `households` — households (name, currency)
+- `household_members` — household membership (any number of people; a user can be in several households at once)
+- `invites` — household invite codes (valid 7 days, reusable — they don't expire after a single join)
+- `password_resets` — one-time password reset tokens (valid 1h)
+- `categories` — expense categories (name, Material Symbols icon, color)
+- `expenses` — expenses (amount, payer, category, date, split type)
+- `expense_shares` — the final split of an expense across household members (always sums to the expense amount regardless of split type)
+- `settlements` — settlement history ("Settle up")

-Saldo per osoba liczone jest jako: `(suma zapłacona przez osobę) − (suma jej udziałów w wydatkach) − (netto rozliczeń)`. Do prezentacji „kto komu ile jest winien" salda są upraszczane zachłannym algorytmem (`backend/src/utils/balance.js: simplifyDebts`), który dla N osób generuje minimalną liczbę przelewów rozliczających wszystkich (zamiast osobnego długu między każdą parą).
+Per-person balance is computed as: `(amount they paid) − (sum of their expense shares) − (net settlements)`. For the "who owes whom" display, balances are simplified with a greedy algorithm (`backend/src/utils/balance.js: simplifyDebts`) that produces the minimum number of transfers to settle everyone (instead of a separate debt between every pair).

-## Wiele gospodarstw — jak to działa
+New rows in `users` get a `language` column (`'pl'` or `'en'`, default `'pl'`); on an existing database this is added automatically on startup via `ALTER TABLE ... ADD COLUMN` — no manual migration, and no existing rows are touched or deleted.

-Użytkownik może należeć do wielu gospodarstw. Ponieważ każdy request do zasobów powiązanych z gospodarstwem (wydatki, kategorie, saldo, statystyki, rozliczenia) musi wiedzieć, którego gospodarstwa dotyczy, frontend wysyła nagłówek `X-Household-Id: <id aktywnego gospodarstwa>` przy każdym takim żądaniu (ustawiany automatycznie przez `frontend/src/household/HouseholdContext.jsx` po przełączeniu gospodarstwa w Ustawieniach). Backend weryfikuje w `requireHousehold` middleware, że zalogowany użytkownik faktycznie jest członkiem podanego gospodarstwa.
+## Multiple households — how it works

-Usunięcie ostatniego członka z gospodarstwa automatycznie kasuje samo gospodarstwo (wraz z całą historią wydatków — kasowanie kaskadowe przez klucze obce). Usunięcie konta użytkownika, który ma współdzieloną historię finansową z innymi (wydatki/udziały/rozliczenia), nie usuwa go fizycznie z bazy (zepsułoby to historię widoczną dla reszty gospodarstwa) — konto jest wtedy anonimizowane (nazwa → „Usunięte konto", e-mail zastąpiony unikalnym nieistniejącym adresem, hasło unieważnione). Świeże konto bez żadnej historii jest usuwane w całości.
+A user can belong to multiple households. Since every request for household-scoped resources (expenses, categories, balance, stats, settlements) needs to know which household it's about, the frontend sends an `X-Household-Id: <active household id>` header on every such request (set automatically by `frontend/src/household/HouseholdContext.jsx` whenever the active household is switched in Settings). The backend's `requireHousehold` middleware verifies the logged-in user is actually a member of the given household.

-## API (skrót)
+Removing the last member from a household automatically deletes the household itself (along with its entire expense history — cascading deletes via foreign keys). Deleting a user account that has shared financial history with others (expenses/shares/settlements) doesn't physically remove it from the database (that would break the history visible to the rest of the household) — instead the account is anonymized (name → "Deleted account", email replaced with a unique non-existent address, password invalidated). A fresh account with no history is deleted outright.

-Wszystkie endpointy poza `/auth/register`, `/auth/login`, `/auth/forgot-password`, `/auth/reset-password` i `/health` wymagają nagłówka `Authorization: Bearer <token>`. Endpointy gospodarstwa/kategorii/wydatków/rozliczeń/statystyk dodatkowo wymagają `X-Household-Id: <id>`.
+## API (overview)

-| Grupa | Endpointy |
+Every endpoint except `/auth/register`, `/auth/login`, `/auth/forgot-password`, `/auth/reset-password` and `/health` requires an `Authorization: Bearer <token>` header. Household/category/expense/settlement/stats endpoints additionally require `X-Household-Id: <id>`.
+
+Error responses are `{ "error": "<code>" }`, where `<code>` is a stable snake_case identifier (e.g. `invalid_credentials`, `household_not_found`) meant to be translated client-side — see [`frontend/src/i18n/locales/en/errors.json`](frontend/src/i18n/locales/en/errors.json) for the full list.
+
+| Group | Endpoints |
 |---|---|
-| Auth | `POST /auth/register`, `/login`, `/change-password`, `/forgot-password`, `/reset-password`, `GET /auth/me`, `PUT /auth/me`, `PUT /auth/me/notifications`, `DELETE /auth/me` |
-| Gospodarstwa | `GET /households` (lista Twoich), `GET/PUT/DELETE /households/:id`, `POST /households`, `POST /households/:id/invite`, `POST /households/join`, `DELETE /households/:id/members/:userId` |
-| Kategorie | `GET/POST/PUT/DELETE /categories[/:id]` |
-| Wydatki | `GET/POST/PUT/DELETE /expenses[/:id]` (filtry: `month`, `categoryId`, `payerId`) |
-| Rozliczenia | `GET/POST /settlements` (POST rozlicza od razu wszystkie uproszczone przelewy) |
-| Statystyki | `GET /stats/balance`, `/summary`, `/monthly`, `/export.csv` |
+| Auth | `POST /auth/register`, `/login`, `/change-password`, `/forgot-password`, `/reset-password`, `GET /auth/me`, `PUT /auth/me`, `PUT /auth/me/notifications`, `PUT /auth/me/language`, `DELETE /auth/me` |
+| Households | `GET /households` (yours), `GET/PUT/DELETE /households/:id`, `POST /households`, `POST /households/:id/invite`, `POST /households/join`, `DELETE /households/:id/members/:userId` |
+| Categories | `GET/POST/PUT/DELETE /categories[/:id]` |
+| Expenses | `GET/POST/PUT/DELETE /expenses[/:id]` (filters: `month`, `categoryId`, `payerId`) |
+| Settlements | `GET/POST /settlements` (POST immediately settles all simplified transfers) |
+| Stats | `GET /stats/balance`, `/summary`, `/monthly`, `/export.csv` |

-## Tryb offline (PWA)
+## Offline mode (PWA)

-Service worker (Workbox, przez `vite-plugin-pwa`) cache'uje powłokę aplikacji i ostatnio pobrane dane GET z API (strategia `NetworkFirst`), więc appka otwiera się i pokazuje dane nawet bez sieci. Nowy wydatek dodany offline trafia do kolejki w IndexedDB (`frontend/src/offline/`) i zostaje automatycznie wysłany po wykryciu powrotu połączenia (`online` event) — widoczny jest wtedy baner z liczbą oczekujących wpisów.
+A service worker (Workbox, via `vite-plugin-pwa`) caches the app shell and recently fetched GET data from the API (`NetworkFirst` strategy), so the app opens and shows data even without a connection. A new expense added offline is queued in IndexedDB (`frontend/src/offline/`) and sent automatically once the connection comes back (`online` event) — a banner then shows the number of pending entries.

-## Znane ograniczenia
+## Known limitations

- Kopiowanie kodu zaproszenia przez `navigator.clipboard` wymaga bezpiecznego kontekstu (HTTPS lub `localhost`) — na zwykłym HTTP w sieci lokalnej przeglądarka może to zablokować; dlatego kod jest zawsze dostępny też jako zaznaczalne pole tekstowe (ręczne kopiowanie zawsze działa).
- `schema.sql` używa `CREATE TABLE IF NOT EXISTS` — dodanie nowej kolumny do istniejącej tabeli w już działającej bazie wymaga ręcznej migracji (`ALTER TABLE`); przy starcie na czystej bazie schemat tworzy się poprawnie od razu.
- Kod zaproszenia do gospodarstwa nie wygasa po pierwszym użyciu (celowo — pozwala zaprosić dowolną liczbę osób tym samym kodem), tylko po czasie (7 dni) lub ręcznym wygenerowaniu nowego w Ustawieniach.
+- Copying the invite code via `navigator.clipboard` requires a secure context (HTTPS or `localhost`) — over plain HTTP on a local network the browser may block it; the code is therefore always also available as a selectable text field (manual copy always works).
+- `schema.sql` uses `CREATE TABLE IF NOT EXISTS` — adding a new column to an existing table on an already-running database requires either a manual migration (`ALTER TABLE`) or startup logic like the one already in place for `users.language` (see `backend/src/db/db.js`); a fresh database gets the current schema immediately.
+- A household invite code doesn't expire after first use (intentionally — it lets you invite any number of people with the same code), only after time (7 days) or a manual regeneration in Settings.

-## Rozwój lokalny (bez Dockera)
+## Local development (without Docker)

-Wymaga Node.js 20+.
+Requires Node.js 20+.

 ```bash
 cd backend && npm install && JWT_SECRET=dev DATABASE_PATH=./data/app.db npm start
-cd frontend && npm install && npm run dev   # serwer dev na :5173, proxy /api -> :3000
+cd frontend && npm install && npm run dev   # dev server on :5173, proxies /api -> :3000
 ```
+
+## License
+
+[MIT](LICENSE)
--- a/backend/package.json
+++ b/backend/package.json
@@ -1,5 +1,5 @@
 {
-  "name": "ktoco-backend",
+  "name": "whowhat-backend",
  "version": "1.0.0",
  "private": true,
  "type": "commonjs",
--- a/backend/src/db/db.js
+++ b/backend/src/db/db.js
@@ -12,14 +12,14 @@ db.pragma('foreign_keys = ON');
 const schema = fs.readFileSync(path.join(__dirname, 'schema.sql'), 'utf8');
 db.exec(schema);

-const DEFAULT_CATEGORIES = [
-  { name: 'Jedzenie', icon: 'shopping_cart', color: '#22c55e' },
-  { name: 'Mieszkanie', icon: 'home', color: '#3b82f6' },
-  { name: 'Rachunki', icon: 'bolt', color: '#f59e0b' },
-  { name: 'Transport', icon: 'directions_car', color: '#8b5cf6' },
-  { name: 'Restauracje', icon: 'restaurant', color: '#ef4444' },
-  { name: 'Rozrywka', icon: 'celebration', color: '#ec4899' },
-  { name: 'Inne', icon: 'inventory_2', color: '#6b7280' },
-];
+// `CREATE TABLE IF NOT EXISTS` (above) only creates the table on a fresh database — it
+// can't add a column to a `users` table that already exists from before this column was
+// introduced. Add it here so upgrading an existing deployment doesn't require a manual
+// migration, and existing users/rows are left untouched (ALTER TABLE ADD COLUMN only adds
+// the new column with its default; it does not delete or modify any existing data).
+const userColumns = db.prepare('PRAGMA table_info(users)').all();
+if (!userColumns.some((c) => c.name === 'language')) {
+  db.exec(`ALTER TABLE users ADD COLUMN language TEXT NOT NULL DEFAULT 'pl'`);
+}

-module.exports = { db, DEFAULT_CATEGORIES };
+module.exports = { db };
--- a/backend/src/db/schema.sql
+++ b/backend/src/db/schema.sql
@@ -5,6 +5,7 @@ CREATE TABLE IF NOT EXISTS users (
  name TEXT NOT NULL,
  avatar_emoji TEXT NOT NULL DEFAULT '🙂',
  email_notifications INTEGER NOT NULL DEFAULT 1,
+  language TEXT NOT NULL DEFAULT 'pl',
  created_at TEXT NOT NULL DEFAULT (datetime('now'))
 );

--- a/backend/src/i18n.js
+++ b/backend/src/i18n.js
@@ -0,0 +1,84 @@
+// Backend-side translations: API error codes (translated client-side, see
+// frontend/src/i18n/locales/*/errors.json), plus content the backend renders itself
+// (transactional emails, CSV export, default category seed) where translation must
+// happen server-side because the client isn't in the loop.
+//
+// To add a language: add a key to SUPPORTED_LANGUAGES and fill in EMAIL_TEMPLATES,
+// DEFAULT_CATEGORIES and CSV_COLUMNS below for that language code.
+
+const SUPPORTED_LANGUAGES = ['pl', 'en'];
+const DEFAULT_LANGUAGE = 'pl';
+
+function normalizeLanguage(lang) {
+  return SUPPORTED_LANGUAGES.includes(lang) ? lang : DEFAULT_LANGUAGE;
+}
+
+const DEFAULT_CATEGORIES = {
+  pl: [
+    { name: 'Jedzenie', icon: 'shopping_cart', color: '#22c55e' },
+    { name: 'Mieszkanie', icon: 'home', color: '#3b82f6' },
+    { name: 'Rachunki', icon: 'bolt', color: '#f59e0b' },
+    { name: 'Transport', icon: 'directions_car', color: '#8b5cf6' },
+    { name: 'Restauracje', icon: 'restaurant', color: '#ef4444' },
+    { name: 'Rozrywka', icon: 'celebration', color: '#ec4899' },
+    { name: 'Inne', icon: 'inventory_2', color: '#6b7280' },
+  ],
+  en: [
+    { name: 'Groceries', icon: 'shopping_cart', color: '#22c55e' },
+    { name: 'Housing', icon: 'home', color: '#3b82f6' },
+    { name: 'Bills', icon: 'bolt', color: '#f59e0b' },
+    { name: 'Transport', icon: 'directions_car', color: '#8b5cf6' },
+    { name: 'Dining out', icon: 'restaurant', color: '#ef4444' },
+    { name: 'Entertainment', icon: 'celebration', color: '#ec4899' },
+    { name: 'Other', icon: 'inventory_2', color: '#6b7280' },
+  ],
+};
+
+function getDefaultCategories(lang) {
+  return DEFAULT_CATEGORIES[normalizeLanguage(lang)];
+}
+
+const EMAIL_TEMPLATES = {
+  pl: {
+    resetPassword: ({ name, resetLink }) => ({
+      subject: 'WhoWhat — reset hasła',
+      text: `Cześć ${name},\n\nAby zresetować hasło, kliknij poniższy link (ważny 1 godzinę):\n${resetLink}\n\nJeśli to nie Ty, zignoruj tę wiadomość.`,
+    }),
+    newExpense: ({ actorName, title, amount }) => ({
+      subject: 'WhoWhat — nowy wydatek',
+      text: `${actorName} dodał(a) nowy wydatek "${title}" na kwotę ${amount}.`,
+    }),
+  },
+  en: {
+    resetPassword: ({ name, resetLink }) => ({
+      subject: 'WhoWhat — password reset',
+      text: `Hi ${name},\n\nTo reset your password, click the link below (valid for 1 hour):\n${resetLink}\n\nIf this wasn't you, just ignore this email.`,
+    }),
+    newExpense: ({ actorName, title, amount }) => ({
+      subject: 'WhoWhat — new expense',
+      text: `${actorName} added a new expense "${title}" for ${amount}.`,
+    }),
+  },
+};
+
+function getEmailTemplates(lang) {
+  return EMAIL_TEMPLATES[normalizeLanguage(lang)];
+}
+
+const CSV_EXPORT = {
+  pl: { header: ['Data', 'Tytuł', 'Kwota', 'Kategoria', 'Płacił', 'Podział'], filename: 'wydatki.csv' },
+  en: { header: ['Date', 'Title', 'Amount', 'Category', 'Paid by', 'Split'], filename: 'expenses.csv' },
+};
+
+function getCsvExport(lang) {
+  return CSV_EXPORT[normalizeLanguage(lang)];
+}
+
+module.exports = {
+  SUPPORTED_LANGUAGES,
+  DEFAULT_LANGUAGE,
+  normalizeLanguage,
+  getDefaultCategories,
+  getEmailTemplates,
+  getCsvExport,
+};
--- a/backend/src/index.js
+++ b/backend/src/index.js
@@ -24,7 +24,7 @@ app.use('/stats', statsRouter);

 app.use((err, req, res, next) => {
  console.error(err);
-  res.status(500).json({ error: 'Wewnętrzny błąd serwera' });
+  res.status(500).json({ error: 'internal_server_error' });
 });

 const PORT = process.env.PORT || 3000;
--- a/backend/src/middleware/auth.js
+++ b/backend/src/middleware/auth.js
@@ -14,20 +14,20 @@ function requireAuth(req, res, next) {
  const header = req.headers.authorization || '';
  const [scheme, token] = header.split(' ');
  if (scheme !== 'Bearer' || !token) {
-    return res.status(401).json({ error: 'Missing bearer token' });
+    return res.status(401).json({ error: 'missing_bearer_token' });
  }
  let payload;
  try {
    payload = jwt.verify(token, JWT_SECRET);
  } catch (err) {
-    return res.status(401).json({ error: 'Invalid or expired token' });
+    return res.status(401).json({ error: 'invalid_or_expired_token' });
  }
  // The JWT signature alone doesn't prove the account still exists (deleted
  // account, or — in dev — a wiped database): reject it the same way so the
  // client logs out instead of misreading "no accounts" as "no household".
  const user = db.prepare('SELECT id FROM users WHERE id = ?').get(payload.sub);
  if (!user) {
-    return res.status(401).json({ error: 'Invalid or expired token' });
+    return res.status(401).json({ error: 'invalid_or_expired_token' });
  }
  req.userId = payload.sub;
  next();
--- a/backend/src/routes/auth.js
+++ b/backend/src/routes/auth.js
@@ -5,6 +5,7 @@ const { v4: uuid } = require('uuid');
 const { db } = require('../db/db');
 const { signToken, requireAuth } = require('../middleware/auth');
 const { sendMail } = require('../utils/mailer');
+const { normalizeLanguage, getEmailTemplates } = require('../i18n');

 const router = express.Router();

@@ -18,21 +19,22 @@ function toPublicUser(user) {
    name: user.name,
    avatarEmoji: user.avatar_emoji,
    emailNotifications: !!user.email_notifications,
+    language: user.language,
  };
 }

 router.post('/register', (req, res) => {
-  const { email, password, name } = req.body || {};
+  const { email, password, name, language } = req.body || {};
  if (!email || !password || !name) {
-    return res.status(400).json({ error: 'email, password i name są wymagane' });
+    return res.status(400).json({ error: 'missing_registration_fields' });
  }
  if (String(password).length < 6) {
-    return res.status(400).json({ error: 'Hasło musi mieć co najmniej 6 znaków' });
+    return res.status(400).json({ error: 'password_too_short' });
  }

  const existing = db.prepare('SELECT id FROM users WHERE email = ?').get(email.toLowerCase());
  if (existing) {
-    return res.status(409).json({ error: 'Konto z tym adresem e-mail już istnieje' });
+    return res.status(409).json({ error: 'email_already_registered' });
  }

  const user = {
@@ -40,12 +42,14 @@ router.post('/register', (req, res) => {
    email: email.toLowerCase(),
    password_hash: bcrypt.hashSync(password, 10),
    name,
+    language: normalizeLanguage(language),
  };
-  db.prepare('INSERT INTO users (id, email, password_hash, name) VALUES (?, ?, ?, ?)').run(
+  db.prepare('INSERT INTO users (id, email, password_hash, name, language) VALUES (?, ?, ?, ?, ?)').run(
    user.id,
    user.email,
    user.password_hash,
-    user.name
+    user.name,
+    user.language
  );

  const created = db.prepare('SELECT * FROM users WHERE id = ?').get(user.id);
@@ -56,12 +60,12 @@ router.post('/register', (req, res) => {
 router.post('/login', (req, res) => {
  const { email, password } = req.body || {};
  if (!email || !password) {
-    return res.status(400).json({ error: 'email i password są wymagane' });
+    return res.status(400).json({ error: 'missing_login_fields' });
  }

  const user = db.prepare('SELECT * FROM users WHERE email = ?').get(email.toLowerCase());
  if (!user || !bcrypt.compareSync(password, user.password_hash)) {
-    return res.status(401).json({ error: 'Nieprawidłowy e-mail lub hasło' });
+    return res.status(401).json({ error: 'invalid_credentials' });
  }

  const token = signToken(user);
@@ -70,14 +74,14 @@ router.post('/login', (req, res) => {

 router.get('/me', requireAuth, (req, res) => {
  const user = db.prepare('SELECT * FROM users WHERE id = ?').get(req.userId);
-  if (!user) return res.status(404).json({ error: 'Użytkownik nie znaleziony' });
+  if (!user) return res.status(404).json({ error: 'user_not_found' });
  res.json({ user: toPublicUser(user) });
 });

 router.put('/me', requireAuth, (req, res) => {
  const { name } = req.body || {};
  if (!name || !name.trim()) {
-    return res.status(400).json({ error: 'name jest wymagane' });
+    return res.status(400).json({ error: 'name_required' });
  }
  db.prepare('UPDATE users SET name = ? WHERE id = ?').run(name.trim(), req.userId);
  const user = db.prepare('SELECT * FROM users WHERE id = ?').get(req.userId);
@@ -109,9 +113,11 @@ router.delete('/me', requireAuth, (req, res) => {
        db.prepare('DELETE FROM households WHERE id = ?').run(householdId);
      }
    }
+    // Not translated: this is a placeholder value stored permanently in the database and
+    // shown to every remaining household member regardless of their own language setting.
    db.prepare(
-      `UPDATE users SET name = 'Usunięte konto', email = ?, password_hash = '', email_notifications = 0 WHERE id = ?`
-    ).run(`deleted-${userId}@ktoco.invalid`, userId);
+      `UPDATE users SET name = 'Deleted account', email = ?, password_hash = '', email_notifications = 0 WHERE id = ?`
+    ).run(`deleted-${userId}@whowhat.invalid`, userId);
  });
  anonymize();

@@ -125,20 +131,27 @@ router.put('/me/notifications', requireAuth, (req, res) => {
  res.json({ user: toPublicUser(user) });
 });

+router.put('/me/language', requireAuth, (req, res) => {
+  const { language } = req.body || {};
+  db.prepare('UPDATE users SET language = ? WHERE id = ?').run(normalizeLanguage(language), req.userId);
+  const user = db.prepare('SELECT * FROM users WHERE id = ?').get(req.userId);
+  res.json({ user: toPublicUser(user) });
+});
+
 router.post('/change-password', requireAuth, (req, res) => {
  const { currentPassword, newPassword } = req.body || {};
  if (!currentPassword || !newPassword) {
-    return res.status(400).json({ error: 'currentPassword i newPassword są wymagane' });
+    return res.status(400).json({ error: 'missing_password_fields' });
  }
  if (String(newPassword).length < 6) {
-    return res.status(400).json({ error: 'Nowe hasło musi mieć co najmniej 6 znaków' });
+    return res.status(400).json({ error: 'password_too_short' });
  }

  const user = db.prepare('SELECT * FROM users WHERE id = ?').get(req.userId);
  if (!bcrypt.compareSync(currentPassword, user.password_hash)) {
    // 400, not 401: the JWT is valid (requireAuth already passed) — this is a form
    // validation failure, not an auth failure, and must not trigger a global session logout.
-    return res.status(400).json({ error: 'Bieżące hasło jest nieprawidłowe' });
+    return res.status(400).json({ error: 'current_password_incorrect' });
  }

  const passwordHash = bcrypt.hashSync(newPassword, 10);
@@ -148,9 +161,9 @@ router.post('/change-password', requireAuth, (req, res) => {

 router.post('/forgot-password', async (req, res) => {
  const { email } = req.body || {};
-  const genericResponse = { message: 'Jeśli konto istnieje, wysłaliśmy e-mail z linkiem do resetu hasła' };
+  const genericResponse = { message: 'If an account exists, a password reset email has been sent' };
  if (!email) {
-    return res.status(400).json({ error: 'email jest wymagany' });
+    return res.status(400).json({ error: 'email_required' });
  }

  const user = db.prepare('SELECT * FROM users WHERE email = ?').get(email.toLowerCase());
@@ -168,11 +181,8 @@ router.post('/forgot-password', async (req, res) => {
  );

  const resetLink = `${FRONTEND_URL}/reset-password?token=${token}`;
-  await sendMail({
-    to: user.email,
-    subject: 'KtoCo — reset hasła',
-    text: `Cześć ${user.name},\n\nAby zresetować hasło, kliknij poniższy link (ważny 1 godzinę):\n${resetLink}\n\nJeśli to nie Ty, zignoruj tę wiadomość.`,
-  });
+  const { subject, text } = getEmailTemplates(user.language).resetPassword({ name: user.name, resetLink });
+  await sendMail({ to: user.email, subject, text });

  res.json(genericResponse);
 });
@@ -180,17 +190,17 @@ router.post('/forgot-password', async (req, res) => {
 router.post('/reset-password', (req, res) => {
  const { token, password } = req.body || {};
  if (!token || !password) {
-    return res.status(400).json({ error: 'token i password są wymagane' });
+    return res.status(400).json({ error: 'missing_reset_fields' });
  }
  if (String(password).length < 6) {
-    return res.status(400).json({ error: 'Hasło musi mieć co najmniej 6 znaków' });
+    return res.status(400).json({ error: 'password_too_short' });
  }

  const reset = db
    .prepare(`SELECT * FROM password_resets WHERE token = ? AND used_at IS NULL AND expires_at > datetime('now')`)
    .get(token);
  if (!reset) {
-    return res.status(400).json({ error: 'Link do resetu hasła jest nieprawidłowy lub wygasł' });
+    return res.status(400).json({ error: 'reset_link_invalid_or_expired' });
  }

  const passwordHash = bcrypt.hashSync(password, 10);
--- a/backend/src/routes/categories.js
+++ b/backend/src/routes/categories.js
@@ -17,7 +17,7 @@ router.get('/', (req, res) => {
 router.post('/', (req, res) => {
  const { name, icon, color } = req.body || {};
  if (!name) {
-    return res.status(400).json({ error: 'Nazwa kategorii jest wymagana' });
+    return res.status(400).json({ error: 'category_name_required' });
  }
  const category = {
    id: uuid(),
@@ -41,7 +41,7 @@ router.put('/:id', (req, res) => {
    .prepare('SELECT * FROM categories WHERE id = ? AND household_id = ?')
    .get(req.params.id, req.household.id);
  if (!existing) {
-    return res.status(404).json({ error: 'Kategoria nie znaleziona' });
+    return res.status(404).json({ error: 'category_not_found' });
  }
  const { name, icon, color } = req.body || {};
  db.prepare('UPDATE categories SET name = ?, icon = ?, color = ? WHERE id = ?').run(
@@ -58,7 +58,7 @@ router.delete('/:id', (req, res) => {
    .prepare('DELETE FROM categories WHERE id = ? AND household_id = ?')
    .run(req.params.id, req.household.id);
  if (result.changes === 0) {
-    return res.status(404).json({ error: 'Kategoria nie znaleziona' });
+    return res.status(404).json({ error: 'category_not_found' });
  }
  res.status(204).end();
 });
--- a/backend/src/routes/expenses.js
+++ b/backend/src/routes/expenses.js
@@ -5,6 +5,7 @@ const { requireAuth } = require('../middleware/auth');
 const { requireHousehold, getMembers } = require('../utils/households');
 const { sendMail } = require('../utils/mailer');
 const { round2 } = require('../utils/balance');
+const { getEmailTemplates } = require('../i18n');

 const router = express.Router();
 router.use(requireAuth, requireHousehold);
@@ -15,7 +16,7 @@ function computeShares(splitType, amount, payerId, members, shares) {
  const memberIds = members.map((m) => m.id);
  if (splitType === 'equal') {
    if (memberIds.length === 0) {
-      throw new Error('Gospodarstwo domowe nie ma członków do podziału wydatku');
+      throw new Error('household_has_no_members');
    }
    const base = Math.floor((amount / memberIds.length) * 100) / 100;
    const shares = Object.fromEntries(memberIds.map((id) => [id, base]));
@@ -28,21 +29,21 @@ function computeShares(splitType, amount, payerId, members, shares) {

  if (splitType === 'exact' || splitType === 'full') {
    if (!shares || typeof shares !== 'object') {
-      throw new Error('Pole "shares" jest wymagane dla wybranego typu podziału');
+      throw new Error('shares_required');
    }
    const sum = Object.values(shares).reduce((acc, v) => acc + Number(v), 0);
    if (Math.abs(sum - amount) > EPSILON) {
-      throw new Error('Suma udziałów musi być równa kwocie wydatku');
+      throw new Error('shares_must_sum_to_amount');
    }
    for (const userId of Object.keys(shares)) {
      if (!memberIds.includes(userId)) {
-        throw new Error('Udział przypisany do osoby spoza gospodarstwa domowego');
+        throw new Error('share_assigned_to_non_member');
      }
    }
    return shares;
  }

-  throw new Error('Nieznany typ podziału');
+  throw new Error('unknown_split_type');
 }

 function attachShares(expense) {
@@ -78,12 +79,12 @@ router.get('/', (req, res) => {
 router.post('/', (req, res) => {
  const { amount, title, categoryId, expenseDate, payerId, splitType, shares } = req.body || {};
  if (!amount || !title || !expenseDate || !payerId || !splitType) {
-    return res.status(400).json({ error: 'amount, title, expenseDate, payerId i splitType są wymagane' });
+    return res.status(400).json({ error: 'missing_expense_fields' });
  }

  const members = getMembers(req.household.id);
  if (!members.some((m) => m.id === payerId)) {
-    return res.status(400).json({ error: 'payerId musi być członkiem gospodarstwa domowego' });
+    return res.status(400).json({ error: 'payer_must_be_member' });
  }

  let computedShares;
@@ -131,17 +132,18 @@ router.post('/', (req, res) => {
  const actor = db.prepare('SELECT name FROM users WHERE id = ?').get(req.userId);
  const notifyTargets = db
    .prepare(
-      `SELECT u.email, u.name FROM users u
+      `SELECT u.email, u.name, u.language FROM users u
       JOIN household_members hm ON hm.user_id = u.id
       WHERE hm.household_id = ? AND u.id != ? AND u.email_notifications = 1`
    )
    .all(req.household.id, req.userId);
  for (const target of notifyTargets) {
-    sendMail({
-      to: target.email,
-      subject: 'KtoCo — nowy wydatek',
-      text: `${actor?.name || 'Ktoś'} dodał(a) nowy wydatek "${expense.title}" na kwotę ${expense.amount.toFixed(2)}.`,
+    const { subject, text } = getEmailTemplates(target.language).newExpense({
+      actorName: actor?.name || (target.language === 'en' ? 'Someone' : 'Ktoś'),
+      title: expense.title,
+      amount: expense.amount.toFixed(2),
    });
+    sendMail({ to: target.email, subject, text });
  }

  const created = db.prepare('SELECT * FROM expenses WHERE id = ?').get(expense.id);
@@ -153,7 +155,7 @@ router.put('/:id', (req, res) => {
    .prepare('SELECT * FROM expenses WHERE id = ? AND household_id = ?')
    .get(req.params.id, req.household.id);
  if (!existing) {
-    return res.status(404).json({ error: 'Wydatek nie znaleziony' });
+    return res.status(404).json({ error: 'expense_not_found' });
  }

  const { amount, title, categoryId, expenseDate, payerId, splitType, shares } = req.body || {};
@@ -163,7 +165,7 @@ router.put('/:id', (req, res) => {
  const finalSplitType = splitType || existing.split_type;

  if (!members.some((m) => m.id === finalPayerId)) {
-    return res.status(400).json({ error: 'payerId musi być członkiem gospodarstwa domowego' });
+    return res.status(400).json({ error: 'payer_must_be_member' });
  }

  let computedShares;
@@ -206,7 +208,7 @@ router.delete('/:id', (req, res) => {
    .prepare('DELETE FROM expenses WHERE id = ? AND household_id = ?')
    .run(req.params.id, req.household.id);
  if (result.changes === 0) {
-    return res.status(404).json({ error: 'Wydatek nie znaleziony' });
+    return res.status(404).json({ error: 'expense_not_found' });
  }
  res.status(204).end();
 });
--- a/backend/src/routes/households.js
+++ b/backend/src/routes/households.js
@@ -1,9 +1,10 @@
 const express = require('express');
 const crypto = require('crypto');
 const { v4: uuid } = require('uuid');
-const { db, DEFAULT_CATEGORIES } = require('../db/db');
+const { db } = require('../db/db');
 const { requireAuth } = require('../middleware/auth');
 const { getHouseholdsForUser, getHouseholdById, isMember, getMembers } = require('../utils/households');
+const { getDefaultCategories } = require('../i18n');

 const router = express.Router();
 router.use(requireAuth);
@@ -32,16 +33,18 @@ router.get('/', (req, res) => {

 router.get('/:id', (req, res) => {
  if (!isMember(req.params.id, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  const household = getHouseholdById(req.params.id);
-  if (!household) return res.status(404).json({ error: 'Gospodarstwo nie znalezione' });
+  if (!household) return res.status(404).json({ error: 'household_not_found' });
  res.json({ household: serializeHousehold(household) });
 });

 router.post('/', (req, res) => {
  const { name, currency } = req.body || {};
-  const household = { id: uuid(), name: name || 'Nasze gospodarstwo', currency: currency || 'PLN' };
+  const requester = db.prepare('SELECT language FROM users WHERE id = ?').get(req.userId);
+  const defaultName = requester?.language === 'en' ? 'Our household' : 'Nasze gospodarstwo';
+  const household = { id: uuid(), name: name || defaultName, currency: currency || 'PLN' };

  const createHousehold = db.transaction(() => {
    db.prepare('INSERT INTO households (id, name, currency) VALUES (?, ?, ?)').run(
@@ -50,7 +53,7 @@ router.post('/', (req, res) => {
      household.currency
    );
    db.prepare('INSERT INTO household_members (household_id, user_id) VALUES (?, ?)').run(household.id, req.userId);
-    for (const cat of DEFAULT_CATEGORIES) {
+    for (const cat of getDefaultCategories(requester?.language)) {
      db.prepare('INSERT INTO categories (id, household_id, name, icon, color) VALUES (?, ?, ?, ?, ?)').run(
        uuid(),
        household.id,
@@ -73,10 +76,10 @@ router.post('/', (req, res) => {

 router.put('/:id', (req, res) => {
  if (!isMember(req.params.id, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  const household = getHouseholdById(req.params.id);
-  if (!household) return res.status(404).json({ error: 'Gospodarstwo nie znalezione' });
+  if (!household) return res.status(404).json({ error: 'household_not_found' });

  const { name, currency } = req.body || {};
  db.prepare('UPDATE households SET name = ?, currency = ? WHERE id = ?').run(
@@ -89,7 +92,7 @@ router.put('/:id', (req, res) => {

 router.post('/:id/invite', (req, res) => {
  if (!isMember(req.params.id, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  db.prepare(`UPDATE invites SET used_at = datetime('now') WHERE household_id = ? AND used_at IS NULL`).run(
    req.params.id
@@ -106,17 +109,17 @@ router.post('/:id/invite', (req, res) => {
 router.post('/join', (req, res) => {
  const { code } = req.body || {};
  if (!code) {
-    return res.status(400).json({ error: 'Kod zaproszenia jest wymagany' });
+    return res.status(400).json({ error: 'invite_code_required' });
  }

  const invite = db
    .prepare(`SELECT * FROM invites WHERE code = ? AND used_at IS NULL AND expires_at > datetime('now')`)
    .get(code.toUpperCase());
  if (!invite) {
-    return res.status(404).json({ error: 'Kod zaproszenia jest nieprawidłowy lub wygasł' });
+    return res.status(404).json({ error: 'invite_code_invalid_or_expired' });
  }
  if (isMember(invite.household_id, req.userId)) {
-    return res.status(409).json({ error: 'Jesteś już członkiem tego gospodarstwa' });
+    return res.status(409).json({ error: 'already_a_member' });
  }

  db.prepare('INSERT INTO household_members (household_id, user_id) VALUES (?, ?)').run(
@@ -129,7 +132,7 @@ router.post('/join', (req, res) => {

 router.delete('/:id/members/:userId', (req, res) => {
  if (!isMember(req.params.id, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  db.prepare('DELETE FROM household_members WHERE household_id = ? AND user_id = ?').run(
    req.params.id,
@@ -145,7 +148,7 @@ router.delete('/:id/members/:userId', (req, res) => {

 router.delete('/:id', (req, res) => {
  if (!isMember(req.params.id, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  db.prepare('DELETE FROM households WHERE id = ?').run(req.params.id);
  res.json({ deleted: true });
--- a/backend/src/routes/settlements.js
+++ b/backend/src/routes/settlements.js
@@ -20,7 +20,7 @@ router.post('/', (req, res) => {
  const current = computeSettlement(req.household.id, members);

  if (current.settled) {
-    return res.status(409).json({ error: 'Jesteście już rozliczeni' });
+    return res.status(409).json({ error: 'already_settled' });
  }

  const insert = db.transaction(() => {
@@ -37,20 +37,20 @@ router.post('/', (req, res) => {

 function validateManualSettlement(req, res, { fromUserId, toUserId, amount }) {
  if (!fromUserId || !toUserId || !amount) {
-    res.status(400).json({ error: 'fromUserId, toUserId i amount są wymagane' });
+    res.status(400).json({ error: 'missing_settlement_fields' });
    return false;
  }
  if (fromUserId === toUserId) {
-    res.status(400).json({ error: 'Płacący i odbiorca muszą być różnymi osobami' });
+    res.status(400).json({ error: 'payer_and_recipient_must_differ' });
    return false;
  }
  if (Number(amount) <= 0) {
-    res.status(400).json({ error: 'Kwota musi być większa od zera' });
+    res.status(400).json({ error: 'amount_must_be_positive' });
    return false;
  }
  const memberIds = getMembers(req.household.id).map((m) => m.id);
  if (!memberIds.includes(fromUserId) || !memberIds.includes(toUserId)) {
-    res.status(400).json({ error: 'Obie osoby muszą być członkami gospodarstwa' });
+    res.status(400).json({ error: 'both_must_be_members' });
    return false;
  }
  return true;
@@ -79,7 +79,7 @@ router.put('/:id', (req, res) => {
    .prepare('SELECT * FROM settlements WHERE id = ? AND household_id = ?')
    .get(req.params.id, req.household.id);
  if (!existing) {
-    return res.status(404).json({ error: 'Rozliczenie nie znalezione' });
+    return res.status(404).json({ error: 'settlement_not_found' });
  }

  const fromUserId = req.body?.fromUserId ?? existing.from_user_id;
@@ -102,7 +102,7 @@ router.delete('/:id', (req, res) => {
    .prepare('DELETE FROM settlements WHERE id = ? AND household_id = ?')
    .run(req.params.id, req.household.id);
  if (result.changes === 0) {
-    return res.status(404).json({ error: 'Rozliczenie nie znalezione' });
+    return res.status(404).json({ error: 'settlement_not_found' });
  }
  res.status(204).end();
 });
--- a/backend/src/routes/stats.js
+++ b/backend/src/routes/stats.js
@@ -3,6 +3,7 @@ const { db } = require('../db/db');
 const { requireAuth } = require('../middleware/auth');
 const { requireHousehold, getMembers } = require('../utils/households');
 const { computeSettlement } = require('../utils/balance');
+const { getCsvExport } = require('../i18n');

 const router = express.Router();
 router.use(requireAuth, requireHousehold);
@@ -75,8 +76,10 @@ router.get('/export.csv', (req, res) => {
    )
    .all(req.household.id);

+  const requester = db.prepare('SELECT language FROM users WHERE id = ?').get(req.userId);
+  const { header, filename } = getCsvExport(requester?.language);
+
  const escape = (v) => `"${String(v ?? '').replace(/"/g, '""')}"`;
-  const header = ['Data', 'Tytuł', 'Kwota', 'Kategoria', 'Płacił', 'Podział'];
  const lines = [header.join(',')];
  for (const r of rows) {
    lines.push(
@@ -85,7 +88,7 @@ router.get('/export.csv', (req, res) => {
  }

  res.setHeader('Content-Type', 'text/csv; charset=utf-8');
-  res.setHeader('Content-Disposition', 'attachment; filename="wydatki.csv"');
+  res.setHeader('Content-Disposition', `attachment; filename="${filename}"`);
  res.send(lines.join('\n'));
 });

--- a/backend/src/utils/households.js
+++ b/backend/src/utils/households.js
@@ -35,14 +35,14 @@ function getMembers(householdId) {
 function requireHousehold(req, res, next) {
  const householdId = req.headers['x-household-id'];
  if (!householdId) {
-    return res.status(400).json({ error: 'Nie wybrano gospodarstwa (brak nagłówka X-Household-Id)' });
+    return res.status(400).json({ error: 'household_not_selected' });
  }
  if (!isMember(householdId, req.userId)) {
-    return res.status(403).json({ error: 'Nie jesteś członkiem tego gospodarstwa' });
+    return res.status(403).json({ error: 'not_a_household_member' });
  }
  const household = getHouseholdById(householdId);
  if (!household) {
-    return res.status(404).json({ error: 'Gospodarstwo nie znalezione' });
+    return res.status(404).json({ error: 'household_not_found' });
  }
  req.household = household;
  next();
--- a/backend/src/utils/mailer.js
+++ b/backend/src/utils/mailer.js
@@ -13,13 +13,13 @@ const transporter = SMTP_HOST

 async function sendMail({ to, subject, text }) {
  if (!transporter) {
-    console.log(`[mailer] SMTP nieskonfigurowane — pomijam wysyłkę do ${to}: "${subject}"`);
+    console.log(`[mailer] SMTP not configured — skipping send to ${to}: "${subject}"`);
    return;
  }
  try {
    await transporter.sendMail({ from: SMTP_FROM || SMTP_USER, to, subject, text });
  } catch (err) {
-    console.error('[mailer] Błąd wysyłki e-mail:', err.message);
+    console.error('[mailer] Failed to send email:', err.message);
  }
 }

--- a/docker-compose.yaml
+++ b/docker-compose.yaml
@@ -5,7 +5,7 @@ services:
      JWT_SECRET: ${JWT_SECRET:-please-change-this-secret-in-.env}
      DATABASE_PATH: /app/data/app.db
      PORT: 3000
-      FRONTEND_URL: ${FRONTEND_URL:-https://ktoco.kzbikowski.pl}
+      FRONTEND_URL: ${FRONTEND_URL:-https://example.com}
      SMTP_HOST: ${SMTP_HOST:-}
      SMTP_PORT: ${SMTP_PORT:-587}
      SMTP_SECURE: ${SMTP_SECURE:-false}
@@ -30,10 +30,10 @@ services:
    labels:
      - traefik.enable=true
      - traefik.docker.network=traefik_public
-      - traefik.http.routers.ktoco.rule=Host(`${DOMAIN:-ktoco.kzbikowski.pl}`)
-      - traefik.http.routers.ktoco.entrypoints=websecure
-      - traefik.http.routers.ktoco.tls.certresolver=tls-resolver
-      - traefik.http.routers.ktoco.tls=true
+      - traefik.http.routers.whowhat.rule=Host(`${DOMAIN:-example.com}`)
+      - traefik.http.routers.whowhat.entrypoints=websecure
+      - traefik.http.routers.whowhat.tls.certresolver=tls-resolver
+      - traefik.http.routers.whowhat.tls=true
    restart: unless-stopped

 networks:
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -4,7 +4,7 @@
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
    <meta name="theme-color" content="#4f46e5" />
-    <title>KtoCo - Wydatki wspólne</title>
+    <title>WhoWhat - Shared Expenses</title>
    <link rel="icon" href="/icons/icon-192.png" />
    <link rel="preconnect" href="https://fonts.googleapis.com" />
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
@@ -14,7 +14,7 @@
    />
    <script>
      (function () {
-        var t = localStorage.getItem('ktoco_theme');
+        var t = localStorage.getItem('whowhat_theme');
        if (t === 'light' || t === 'dark') {
          document.documentElement.setAttribute('data-theme', t);
        }
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -1,5 +1,5 @@
 {
-  "name": "ktoco-frontend",
+  "name": "whowhat-frontend",
  "version": "1.0.0",
  "private": true,
  "type": "module",
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -1,6 +1,7 @@
 import { Routes, Route, Navigate, Outlet } from 'react-router-dom';
 import { useAuth } from './auth/AuthContext.jsx';
 import { useHouseholdContext } from './household/HouseholdContext.jsx';
+import { useTranslation } from './i18n/I18nContext.jsx';
 import Login from './pages/Login.jsx';
 import Register from './pages/Register.jsx';
 import ForgotPassword from './pages/ForgotPassword.jsx';
@@ -26,7 +27,8 @@ function RequireAuth() {

 function RequireHousehold() {
  const { households, householdsLoading } = useHouseholdContext();
-  if (householdsLoading) return <div className="page-loading">Ładowanie…</div>;
+  const { t } = useTranslation();
+  if (householdsLoading) return <div className="page-loading">{t('app.loading')}</div>;
  if (households.length === 0) return <Navigate to="/onboarding" replace />;
  return <Outlet />;
 }
--- a/frontend/src/api/client.js
+++ b/frontend/src/api/client.js
@@ -1,5 +1,5 @@
-const TOKEN_KEY = 'ktoco_token';
-const ACTIVE_HOUSEHOLD_KEY = 'ktoco_active_household';
+const TOKEN_KEY = 'whowhat_token';
+const ACTIVE_HOUSEHOLD_KEY = 'whowhat_active_household';

 export function getToken() {
  return localStorage.getItem(TOKEN_KEY);
@@ -15,7 +15,7 @@ export function setToken(token) {
 // misreading "no household" and bouncing to /onboarding.
 export function clearInvalidSession() {
  setToken(null);
-  window.dispatchEvent(new Event('ktoco:auth-invalid'));
+  window.dispatchEvent(new Event('whowhat:auth-invalid'));
 }

 export function getActiveHouseholdId() {
@@ -60,7 +60,7 @@ export async function apiFetch(path, options = {}) {
  const data = isJson ? await res.json() : await res.text();

  if (!res.ok) {
-    throw new ApiError(isJson ? data.error || 'Wystąpił błąd' : data, res.status);
+    throw new ApiError(isJson ? data.error || 'generic' : data, res.status);
  }
  return data;
 }
--- a/frontend/src/api/queries.js
+++ b/frontend/src/api/queries.js
@@ -200,6 +200,14 @@ export function useUpdateNotifications() {
  });
 }

+export function useUpdateAccountLanguage() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (language) => api.put('/auth/me/language', { language }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['me'] }),
+  });
+}
+
 export function useChangePassword() {
  return useMutation({
    mutationFn: ({ currentPassword, newPassword }) =>
--- a/frontend/src/auth/AuthContext.jsx
+++ b/frontend/src/auth/AuthContext.jsx
@@ -1,12 +1,14 @@
 import { createContext, useContext, useEffect, useState, useCallback } from 'react';
 import { useQueryClient } from '@tanstack/react-query';
 import { api, getToken, setToken } from '../api/client.js';
+import { useTranslation } from '../i18n/I18nContext.jsx';

-const USER_KEY = 'ktoco_user';
+const USER_KEY = 'whowhat_user';
 const AuthContext = createContext(null);

 export function AuthProvider({ children }) {
  const qc = useQueryClient();
+  const { language, setLanguage } = useTranslation();
  const [user, setUser] = useState(() => {
    const raw = localStorage.getItem(USER_KEY);
    return raw ? JSON.parse(raw) : null;
@@ -24,19 +26,22 @@ export function AuthProvider({ children }) {
      const data = await api.post('/auth/login', { email, password });
      setToken(data.token);
      persistUser(data.user);
+      if (data.user.language) setLanguage(data.user.language);
      return data.user;
    },
-    [persistUser]
+    [persistUser, setLanguage]
  );

  const register = useCallback(
    async (email, password, name) => {
-      const data = await api.post('/auth/register', { email, password, name });
+      // The account is created with whatever language the visitor is currently browsing
+      // in, so e.g. reset-password emails match what they saw at signup.
+      const data = await api.post('/auth/register', { email, password, name, language });
      setToken(data.token);
      persistUser(data.user);
      return data.user;
    },
-    [persistUser]
+    [persistUser, language]
  );

  const logout = useCallback(() => {
@@ -56,8 +61,8 @@ export function AuthProvider({ children }) {
      persistUser(null);
      qc.clear();
    }
-    window.addEventListener('ktoco:auth-invalid', handleAuthInvalid);
-    return () => window.removeEventListener('ktoco:auth-invalid', handleAuthInvalid);
+    window.addEventListener('whowhat:auth-invalid', handleAuthInvalid);
+    return () => window.removeEventListener('whowhat:auth-invalid', handleAuthInvalid);
  }, [persistUser, qc]);

  return (
--- a/frontend/src/components/BottomNav.jsx
+++ b/frontend/src/components/BottomNav.jsx
@@ -1,17 +1,20 @@
 import { NavLink } from 'react-router-dom';
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';
+
+export default function BottomNav() {
+  const { t } = useTranslation();

  const items = [
-  { to: '/', label: 'Start', icon: 'home', end: true },
-  { to: '/history', label: 'Historia', icon: 'receipt_long' },
+    { to: '/', label: t('bottomNav.home'), icon: 'home', end: true },
+    { to: '/history', label: t('bottomNav.history'), icon: 'receipt_long' },
  ];

  const rightItems = [
-  { to: '/settlements', label: 'Rozliczenia', icon: 'payments' },
-  { to: '/stats', label: 'Statystyki', icon: 'bar_chart' },
+    { to: '/settlements', label: t('bottomNav.settlements'), icon: 'payments' },
+    { to: '/stats', label: t('bottomNav.stats'), icon: 'bar_chart' },
  ];

-export default function BottomNav() {
  return (
    <nav className="bottom-nav">
      {items.map((item) => (
@@ -20,7 +23,7 @@ export default function BottomNav() {
          <span>{item.label}</span>
        </NavLink>
      ))}
-      <NavLink to="/add" className="fab" aria-label="Dodaj wydatek">
+      <NavLink to="/add" className="fab" aria-label={t('bottomNav.addExpense')}>
        <Icon name="add" />
      </NavLink>
      {rightItems.map((item) => (
--- a/frontend/src/components/CategoryPieChart.jsx
+++ b/frontend/src/components/CategoryPieChart.jsx
@@ -1,12 +1,14 @@
 import { PieChart, Pie, Cell, Tooltip, ResponsiveContainer, Legend } from 'recharts';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function CategoryPieChart({ data, currency }) {
+  const { t } = useTranslation();
  const chartData = data
    .filter((d) => d.total > 0)
-    .map((d) => ({ name: d.name || 'Bez kategorii', value: d.total, color: d.color || '#6b7280' }));
+    .map((d) => ({ name: d.name || t('charts.noCategory'), value: d.total, color: d.color || '#6b7280' }));

  if (chartData.length === 0) {
-    return <p className="empty-state">Brak wydatków w tym miesiącu</p>;
+    return <p className="empty-state">{t('charts.noExpensesThisMonth')}</p>;
  }

  return (
--- a/frontend/src/components/ConfirmDialogProvider.jsx
+++ b/frontend/src/components/ConfirmDialogProvider.jsx
@@ -1,23 +1,28 @@
 import { createContext, useCallback, useContext, useState } from 'react';
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 const ConfirmContext = createContext(null);

 export function ConfirmProvider({ children }) {
  const [state, setState] = useState(null);
+  const { t } = useTranslation();

-  const confirmAction = useCallback((options) => {
+  const confirmAction = useCallback(
+    (options) => {
      return new Promise((resolve) => {
        setState({
-        title: options.title || 'Czy na pewno?',
+          title: options.title || t('confirmDialog.defaultTitle'),
          message: options.message || '',
-        confirmLabel: options.confirmLabel || 'Usuń',
-        cancelLabel: options.cancelLabel || 'Anuluj',
+          confirmLabel: options.confirmLabel || t('confirmDialog.defaultConfirm'),
+          cancelLabel: options.cancelLabel || t('confirmDialog.defaultCancel'),
          danger: options.danger !== false,
          resolve,
        });
      });
-  }, []);
+    },
+    [t]
+  );

  function handle(result) {
    state?.resolve(result);
--- a/frontend/src/components/ErrorBoundary.jsx
+++ b/frontend/src/components/ErrorBoundary.jsx
@@ -1,4 +1,13 @@
 import { Component } from 'react';
+import en from '../i18n/locales/en/errorBoundary.json';
+import pl from '../i18n/locales/pl/errorBoundary.json';
+
+// Sits above I18nProvider so it must keep working even if the provider itself throws —
+// it can't rely on useTranslation() and instead reads the persisted language directly.
+function currentTranslations() {
+  const lang = localStorage.getItem('whowhat_language');
+  return lang === 'en' ? en : pl;
+}

 export default class ErrorBoundary extends Component {
  state = { error: null };
@@ -8,17 +17,18 @@ export default class ErrorBoundary extends Component {
  }

  componentDidCatch(error, info) {
-    console.error('Nieobsłużony błąd renderowania:', error, info);
+    console.error('Unhandled render error:', error, info);
  }

  render() {
    if (this.state.error) {
+      const t = currentTranslations();
      return (
        <div className="auth-page">
-          <h1>Coś poszło nie tak</h1>
-          <p className="auth-subtitle">Wystąpił nieoczekiwany błąd. Spróbuj odświeżyć stronę.</p>
+          <h1>{t.title}</h1>
+          <p className="auth-subtitle">{t.subtitle}</p>
          <button className="btn-primary" onClick={() => window.location.reload()}>
-            Odśwież
+            {t.refresh}
          </button>
        </div>
      );
--- a/frontend/src/components/ExpenseEditModal.jsx
+++ b/frontend/src/components/ExpenseEditModal.jsx
@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { useUpdateExpense, useDeleteExpense } from '../api/queries.js';
 import { useConfirm } from './ConfirmDialogProvider.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';
 import PayerToggle from './PayerToggle.jsx';
 import SplitSelector from './SplitSelector.jsx';
 import Icon from './Icon.jsx';
@@ -9,6 +10,7 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
  const updateExpense = useUpdateExpense();
  const deleteExpense = useDeleteExpense();
  const confirmDialog = useConfirm();
+  const { t, tError } = useTranslation();

  const [amount, setAmount] = useState(String(expense.amount));
  const [title, setTitle] = useState(expense.title);
@@ -50,15 +52,15 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
      });
      onClose();
    } catch (err) {
-      setError(err.message);
+      setError(tError(err));
    }
  }

  async function handleDelete() {
    const ok = await confirmDialog({
-      title: 'Usunąć ten wydatek?',
-      message: 'Tej operacji nie da się cofnąć.',
-      confirmLabel: 'Usuń',
+      title: t('expenseEditModal.deleteConfirmTitle'),
+      message: t('expenseEditModal.deleteConfirmMessage'),
+      confirmLabel: t('common.delete'),
    });
    if (!ok) return;
    await deleteExpense.mutateAsync(expense.id);
@@ -69,7 +71,7 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
    <div className="modal-overlay" onClick={onClose}>
      <div className="modal-sheet" onClick={(e) => e.stopPropagation()}>
        <div className="modal-header">
-          <h3>Edytuj wydatek</h3>
+          <h3>{t('expenseEditModal.title')}</h3>
          <button className="icon-btn" onClick={onClose}><Icon name="close" /></button>
        </div>

@@ -84,12 +86,12 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
          />

          <div className="field">
-            <label>Tytuł</label>
+            <label>{t('expenseEditModal.titleFieldLabel')}</label>
            <input type="text" value={title} onChange={(e) => setTitle(e.target.value)} />
          </div>

          <div className="field">
-            <label>Kategoria</label>
+            <label>{t('expenseEditModal.categoryLabel')}</label>
            <div className="category-grid">
              {(categories || []).map((c) => (
                <div
@@ -105,7 +107,7 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
          </div>

          <div className="field">
-            <label>Kto płacił?</label>
+            <label>{t('expenseEditModal.whoPaidLabel')}</label>
            <PayerToggle members={members} payerId={payerId} onChange={setPayerId} />
          </div>

@@ -121,7 +123,7 @@ export default function ExpenseEditModal({ expense, members, categories, currenc
          />

          <div className="field">
-            <label>Data</label>
+            <label>{t('expenseEditModal.dateLabel')}</label>
            <input type="date" value={expenseDate} onChange={(e) => setExpenseDate(e.target.value)} />
          </div>

@@ -129,10 +131,10 @@ export default function ExpenseEditModal({ expense, members, categories, currenc

          <div className="modal-actions">
            <button className="btn-danger" onClick={handleDelete} disabled={deleteExpense.isPending}>
-              <Icon name="delete" /> Usuń
+              <Icon name="delete" /> {t('common.delete')}
            </button>
            <button className="btn-primary" onClick={handleSave} disabled={updateExpense.isPending}>
-              Zapisz
+              {t('common.save')}
            </button>
          </div>
        </div>
--- a/frontend/src/components/ExpenseListItem.jsx
+++ b/frontend/src/components/ExpenseListItem.jsx
@@ -1,7 +1,9 @@
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function ExpenseListItem({ expense, category, payer, currentUserId, currency, onClick }) {
  const myShare = expense.shares.find((s) => s.user_id === currentUserId)?.share_amount ?? 0;
+  const { t } = useTranslation();

  return (
    <div className="expense-item" onClick={onClick}>
@@ -19,7 +21,7 @@ export default function ExpenseListItem({ expense, category, payer, currentUserI
      </div>
      <div className="amount-col">
        <div className="amount">{expense.amount.toFixed(2)} {currency}</div>
-        <div className="share">Twoja część: {myShare.toFixed(2)} {currency}</div>
+        <div className="share">{t('expenseListItem.yourShare')}: {myShare.toFixed(2)} {currency}</div>
      </div>
    </div>
  );
--- a/frontend/src/components/InstallBanner.jsx
+++ b/frontend/src/components/InstallBanner.jsx
@@ -1,12 +1,14 @@
 import { useState } from 'react';
 import { useInstallPrompt } from '../pwa/useInstallPrompt.js';
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

-const DISMISS_KEY = 'ktoco_install_dismissed';
+const DISMISS_KEY = 'whowhat_install_dismissed';

 export default function InstallBanner() {
  const { canInstall, isIOS, promptInstall } = useInstallPrompt();
  const [dismissed, setDismissed] = useState(() => localStorage.getItem(DISMISS_KEY) === '1');
+  const { t } = useTranslation();

  if (dismissed || (!canInstall && !isIOS)) return null;

@@ -20,20 +22,20 @@ export default function InstallBanner() {
      <Icon name="install_mobile" />
      <div className="install-banner-text">
        {canInstall ? (
-          <span>Zainstaluj KtoCo jako aplikację na telefonie</span>
+          <span>{t('installBanner.installPrompt', { appName: t('common.appName') })}</span>
        ) : (
          <span>
-            Dodaj KtoCo do ekranu głównego: dotknij <Icon name="ios_share" className="inline-icon" /> Udostępnij, a
-            potem „Dodaj do ekranu początkowego”
+            {t('installBanner.installIosPrompt', { appName: t('common.appName') })}{' '}
+            <Icon name="ios_share" className="inline-icon" />
          </span>
        )}
      </div>
      {canInstall && (
        <button className="btn-primary install-banner-btn" onClick={promptInstall}>
-          Zainstaluj
+          {t('installBanner.install')}
        </button>
      )}
-      <button className="icon-btn" onClick={handleDismiss} aria-label="Zamknij">
+      <button className="icon-btn" onClick={handleDismiss} aria-label={t('installBanner.close')}>
        <Icon name="close" />
      </button>
    </div>
--- a/frontend/src/components/LanguageSwitcher.jsx
+++ b/frontend/src/components/LanguageSwitcher.jsx
@@ -0,0 +1,27 @@
+import { useTranslation } from '../i18n/I18nContext.jsx';
+import { useAuth } from '../auth/AuthContext.jsx';
+import { useUpdateAccountLanguage } from '../api/queries.js';
+
+export default function LanguageSwitcher() {
+  const { language, setLanguage, languages, t } = useTranslation();
+  const { isAuthenticated } = useAuth();
+  const updateAccountLanguage = useUpdateAccountLanguage();
+
+  function handleSelect(code) {
+    setLanguage(code);
+    if (isAuthenticated) updateAccountLanguage.mutate(code);
+  }
+
+  return (
+    <div className="card">
+      <h3>{t('languageSwitcher.label')}</h3>
+      <div className="theme-toggle">
+        {languages.map((l) => (
+          <button key={l.code} className={language === l.code ? 'tab active' : 'tab'} onClick={() => handleSelect(l.code)}>
+            <span>{l.label}</span>
+          </button>
+        ))}
+      </div>
+    </div>
+  );
+}
--- a/frontend/src/components/MonthlyBarChart.jsx
+++ b/frontend/src/components/MonthlyBarChart.jsx
@@ -1,8 +1,10 @@
 import { BarChart, Bar, XAxis, YAxis, Tooltip, ResponsiveContainer, CartesianGrid } from 'recharts';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function MonthlyBarChart({ data, currency }) {
+  const { t } = useTranslation();
  if (!data || data.length === 0) {
-    return <p className="empty-state">Brak danych historycznych</p>;
+    return <p className="empty-state">{t('charts.noHistoricalData')}</p>;
  }

  return (
--- a/frontend/src/components/OfflineBanner.jsx
+++ b/frontend/src/components/OfflineBanner.jsx
@@ -1,14 +1,16 @@
 import { useOnlineSync } from '../offline/useOnlineSync.js';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function OfflineBanner() {
  const { isOnline, pendingCount } = useOnlineSync();
+  const { t } = useTranslation();

  if (isOnline && pendingCount === 0) return null;

  return (
    <div className={`offline-banner ${isOnline ? 'offline-banner--syncing' : ''}`}>
-      {!isOnline && <span>Brak połączenia — wydatki zapisują się lokalnie</span>}
-      {isOnline && pendingCount > 0 && <span>Synchronizowanie {pendingCount} wydatków…</span>}
+      {!isOnline && <span>{t('offlineBanner.offline')}</span>}
+      {isOnline && pendingCount > 0 && <span>{t('offlineBanner.syncing', { count: pendingCount })}</span>}
    </div>
  );
 }
--- a/frontend/src/components/PasswordField.jsx
+++ b/frontend/src/components/PasswordField.jsx
@@ -1,8 +1,10 @@
 import { useState } from 'react';
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function PasswordField({ value, onChange, placeholder }) {
  const [visible, setVisible] = useState(false);
+  const { t } = useTranslation();

  return (
    <div className="password-field">
@@ -17,7 +19,7 @@ export default function PasswordField({ value, onChange, placeholder }) {
        type="button"
        className="password-toggle"
        onClick={() => setVisible((v) => !v)}
-        aria-label={visible ? 'Ukryj hasło' : 'Pokaż hasło'}
+        aria-label={visible ? t('passwordField.hide') : t('passwordField.show')}
      >
        <Icon name={visible ? 'visibility_off' : 'visibility'} />
      </button>
--- a/frontend/src/components/PayerComparisonChart.jsx
+++ b/frontend/src/components/PayerComparisonChart.jsx
@@ -1,15 +1,17 @@
 import { BarChart, Bar, XAxis, YAxis, Tooltip, ResponsiveContainer, CartesianGrid, Cell } from 'recharts';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 const COLORS = ['#4f46e5', '#f59e0b', '#22c55e', '#ef4444', '#8b5cf6', '#06b6d4', '#ec4899', '#84cc16'];

 export default function PayerComparisonChart({ members, byShare, currency }) {
+  const { t } = useTranslation();
  const data = members.map((m) => ({
    name: m.name,
    value: byShare.find((s) => s.userId === m.id)?.total || 0,
  }));

  if (data.every((d) => d.value === 0)) {
-    return <p className="empty-state">Brak wydatków w tym miesiącu</p>;
+    return <p className="empty-state">{t('charts.noExpensesThisMonth')}</p>;
  }

  return (
--- a/frontend/src/components/SettingsButton.jsx
+++ b/frontend/src/components/SettingsButton.jsx
@@ -1,9 +1,11 @@
 import { NavLink } from 'react-router-dom';
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 export default function SettingsButton() {
+  const { t } = useTranslation();
  return (
-    <NavLink to="/settings" className="top-settings-btn" aria-label="Ustawienia">
+    <NavLink to="/settings" className="top-settings-btn" aria-label={t('settingsButton.settings')}>
      <Icon name="settings" />
    </NavLink>
  );
--- a/frontend/src/components/SettlementEditModal.jsx
+++ b/frontend/src/components/SettlementEditModal.jsx
@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { useUpdateSettlement, useDeleteSettlement } from '../api/queries.js';
 import { useConfirm } from './ConfirmDialogProvider.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';
 import PayerToggle from './PayerToggle.jsx';
 import Icon from './Icon.jsx';

@@ -8,6 +9,7 @@ export default function SettlementEditModal({ settlement, members, currency, onC
  const updateSettlement = useUpdateSettlement();
  const deleteSettlement = useDeleteSettlement();
  const confirmDialog = useConfirm();
+  const { t, tError } = useTranslation();

  const [fromUserId, setFromUserId] = useState(settlement.from_user_id);
  const [toUserId, setToUserId] = useState(settlement.to_user_id);
@@ -27,22 +29,22 @@ export default function SettlementEditModal({ settlement, members, currency, onC
  async function handleSave() {
    setError('');
    if (!fromUserId || !toUserId || !amount || Number(amount) <= 0) {
-      setError('Wybierz obie osoby i podaj kwotę większą od zera');
+      setError(t('settlements.selectBothAndAmount'));
      return;
    }
    try {
      await updateSettlement.mutateAsync({ id: settlement.id, fromUserId, toUserId, amount: Number(amount) });
      onClose();
    } catch (err) {
-      setError(err.message);
+      setError(tError(err));
    }
  }

  async function handleDelete() {
    const ok = await confirmDialog({
-      title: 'Usunąć to rozliczenie?',
-      message: 'Saldo zostanie przeliczone tak, jakby ta płatność nigdy nie miała miejsca.',
-      confirmLabel: 'Usuń',
+      title: t('settlementEditModal.deleteConfirmTitle'),
+      message: t('settlementEditModal.deleteConfirmMessage'),
+      confirmLabel: t('common.delete'),
    });
    if (!ok) return;
    await deleteSettlement.mutateAsync(settlement.id);
@@ -53,17 +55,17 @@ export default function SettlementEditModal({ settlement, members, currency, onC
    <div className="modal-overlay" onClick={onClose}>
      <div className="modal-sheet" onClick={(e) => e.stopPropagation()}>
        <div className="modal-header">
-          <h3>Edytuj rozliczenie</h3>
+          <h3>{t('settlementEditModal.title')}</h3>
          <button className="icon-btn" onClick={onClose}><Icon name="close" /></button>
        </div>

        <div className="expense-form">
          <div className="field">
-            <label>Kto płacił?</label>
+            <label>{t('settlementEditModal.whoPaidLabel')}</label>
            <PayerToggle members={members} payerId={fromUserId} onChange={handleFromChange} />
          </div>
          <div className="field">
-            <label>Komu?</label>
+            <label>{t('settlementEditModal.toWhomLabel')}</label>
            <PayerToggle members={toOptions} payerId={toUserId} onChange={setToUserId} />
          </div>
          <input
@@ -79,10 +81,10 @@ export default function SettlementEditModal({ settlement, members, currency, onC

          <div className="modal-actions">
            <button className="btn-danger" onClick={handleDelete} disabled={deleteSettlement.isPending}>
-              <Icon name="delete" /> Usuń
+              <Icon name="delete" /> {t('common.delete')}
            </button>
            <button className="btn-primary" onClick={handleSave} disabled={updateSettlement.isPending}>
-              Zapisz
+              {t('common.save')}
            </button>
          </div>
        </div>
--- a/frontend/src/components/SettlementListItem.jsx
+++ b/frontend/src/components/SettlementListItem.jsx
@@ -1,10 +1,12 @@
 import Icon from './Icon.jsx';
+import { useTranslation } from '../i18n/I18nContext.jsx';

 function memberName(members, id) {
  return members.find((m) => m.id === id)?.name || '—';
 }

 export default function SettlementListItem({ settlement, members, currency, onClick }) {
+  const { t } = useTranslation();
  return (
    <div className="expense-item settlement-item" onClick={onClick}>
      <div className="cat-icon settlement-icon">
@@ -17,7 +19,7 @@ export default function SettlementListItem({ settlement, members, currency, onCl
        <div className="meta">
          <span>{settlement.settled_at.slice(0, 10)}</span>
          <span>·</span>
-          <span>Rozliczenie</span>
+          <span>{t('settlementListItem.settlement')}</span>
        </div>
      </div>
      <div className="amount-col">
--- a/frontend/src/components/SplitSelector.jsx
+++ b/frontend/src/components/SplitSelector.jsx
@@ -1,10 +1,5 @@
 import { useEffect } from 'react';
-
-const OPTIONS = [
-  { value: 'equal', label: 'Po równo (50/50)' },
-  { value: 'exact', label: 'Dokładny podział' },
-  { value: 'full', label: 'Całość na jedną osobę' },
-];
+import { useTranslation } from '../i18n/I18nContext.jsx';

 function round2(n) {
  return Math.round(n * 100) / 100;
@@ -25,6 +20,7 @@ function isEmptyShares(shares, members) {
 }

 function ExactSplitEditor({ members, amount, exactShares, onExactSharesChange }) {
+  const { t } = useTranslation();
  const total = Number(amount) || 0;
  const editableMembers = members.slice(0, -1);
  const lastMember = members[members.length - 1];
@@ -81,7 +77,7 @@ function ExactSplitEditor({ members, amount, exactShares, onExactSharesChange })
            <input type="number" value={bValue.toFixed(2)} readOnly disabled />
          </div>
        </div>
-        {overAllocated && <p className="form-error">Suma udziałów przekracza kwotę wydatku</p>}
+        {overAllocated && <p className="form-error">{t('splitSelector.overAllocated')}</p>}
      </div>
    );
  }
@@ -102,11 +98,11 @@ function ExactSplitEditor({ members, amount, exactShares, onExactSharesChange })
      ))}
      {lastMember && (
        <div className="field">
-          <label>{lastMember.name} (wyliczane automatycznie)</label>
+          <label>{lastMember.name} {t('splitSelector.autoCalculated')}</label>
          <input type="number" value={lastValue.toFixed(2)} readOnly disabled />
        </div>
      )}
-      {overAllocated && <p className="form-error">Suma udziałów przekracza kwotę wydatku</p>}
+      {overAllocated && <p className="form-error">{t('splitSelector.overAllocated')}</p>}
    </div>
  );
 }
@@ -121,9 +117,16 @@ export default function SplitSelector({
  fullOwedBy,
  onFullOwedByChange,
 }) {
+  const { t } = useTranslation();
+  const OPTIONS = [
+    { value: 'equal', label: t('splitSelector.equal') },
+    { value: 'exact', label: t('splitSelector.exact') },
+    { value: 'full', label: t('splitSelector.full') },
+  ];
+
  return (
    <div className="field">
-      <label>Jak dzielimy wydatek?</label>
+      <label>{t('splitSelector.label')}</label>
      <div className="split-options">
        {OPTIONS.map((opt) => (
          <div
@@ -153,7 +156,7 @@ export default function SplitSelector({
                    className={`toggle-btn ${fullOwedBy === m.id ? 'selected' : ''}`}
                    onClick={() => onFullOwedByChange(m.id)}
                  >
-                    {m.name} winien(-na) całość
+                    {t('splitSelector.owesFullAmount', { name: m.name })}
                  </button>
                ))}
              </div>
--- a/frontend/src/i18n/I18nContext.jsx
+++ b/frontend/src/i18n/I18nContext.jsx
@@ -0,0 +1,98 @@
+import { createContext, useCallback, useContext, useEffect, useMemo, useState } from 'react';
+import { SUPPORTED_LANGUAGES, DEFAULT_LANGUAGE } from './languages.js';
+
+const LANGUAGE_KEY = 'whowhat_language';
+const SUPPORTED_CODES = SUPPORTED_LANGUAGES.map((l) => l.code);
+
+// Every locales/<lang>/<namespace>.json file is bundled here at build time and merged
+// into one dictionary per language, keyed by namespace (the filename without extension).
+const modules = import.meta.glob('./locales/*/*.json', { eager: true });
+
+function buildDictionaries() {
+  const dictionaries = {};
+  for (const path in modules) {
+    const match = path.match(/\.\/locales\/([^/]+)\/([^/]+)\.json$/);
+    if (!match) continue;
+    const [, lang, namespace] = match;
+    dictionaries[lang] = dictionaries[lang] || {};
+    dictionaries[lang][namespace] = modules[path].default;
+  }
+  return dictionaries;
+}
+
+const DICTIONARIES = buildDictionaries();
+
+function resolve(dictionary, key) {
+  const [namespace, ...rest] = key.split('.');
+  let node = dictionary?.[namespace];
+  for (const part of rest) {
+    if (node == null) return undefined;
+    node = node[part];
+  }
+  return typeof node === 'string' ? node : undefined;
+}
+
+function interpolate(template, params) {
+  if (!params) return template;
+  return template.replace(/\{\{(\w+)\}\}/g, (match, name) => (name in params ? String(params[name]) : match));
+}
+
+function detectInitialLanguage() {
+  const stored = localStorage.getItem(LANGUAGE_KEY);
+  if (stored && SUPPORTED_CODES.includes(stored)) return stored;
+  const browserLang = (navigator.language || '').slice(0, 2).toLowerCase();
+  if (SUPPORTED_CODES.includes(browserLang)) return browserLang;
+  return DEFAULT_LANGUAGE;
+}
+
+const I18nContext = createContext(null);
+
+export function I18nProvider({ children }) {
+  const [language, setLanguageState] = useState(detectInitialLanguage);
+
+  useEffect(() => {
+    document.documentElement.setAttribute('lang', language);
+  }, [language]);
+
+  const setLanguage = useCallback((lang) => {
+    if (!SUPPORTED_CODES.includes(lang)) return;
+    localStorage.setItem(LANGUAGE_KEY, lang);
+    setLanguageState(lang);
+  }, []);
+
+  const t = useCallback(
+    (key, params) => {
+      const value =
+        resolve(DICTIONARIES[language], key) ??
+        resolve(DICTIONARIES[DEFAULT_LANGUAGE], key) ??
+        resolve(DICTIONARIES.en, key);
+      if (value === undefined) return key;
+      return interpolate(value, params);
+    },
+    [language]
+  );
+
+  // Translates an error code coming from the API (ApiError#message). Falls back to the
+  // raw code (or a generic message) if the backend ever sends something not in errors.json.
+  const tError = useCallback(
+    (err) => {
+      const code = err?.message;
+      if (!code) return t('errors.generic');
+      return resolve(DICTIONARIES[language], `errors.${code}`) ?? code;
+    },
+    [language, t]
+  );
+
+  const value = useMemo(
+    () => ({ language, setLanguage, languages: SUPPORTED_LANGUAGES, t, tError }),
+    [language, setLanguage, t, tError]
+  );
+
+  return <I18nContext.Provider value={value}>{children}</I18nContext.Provider>;
+}
+
+export function useTranslation() {
+  const ctx = useContext(I18nContext);
+  if (!ctx) throw new Error('useTranslation must be used within I18nProvider');
+  return ctx;
+}
--- a/frontend/src/i18n/languages.js
+++ b/frontend/src/i18n/languages.js
@@ -0,0 +1,13 @@
+// Registry of supported UI languages.
+//
+// To add a new language:
+//   1. Create frontend/src/i18n/locales/<code>/ and add a .json file per namespace,
+//      mirroring the files in locales/en/ (same filenames, same keys).
+//   2. Add an entry below with the language's own name (shown in the language switcher).
+// No other code changes are needed — I18nContext.jsx discovers locale files automatically.
+export const SUPPORTED_LANGUAGES = [
+  { code: 'pl', label: 'Polski' },
+  { code: 'en', label: 'English' },
+];
+
+export const DEFAULT_LANGUAGE = 'pl';
--- a/frontend/src/i18n/locales/en/addExpense.json
+++ b/frontend/src/i18n/locales/en/addExpense.json
@@ -0,0 +1,15 @@
+{
+  "pageTitle": "Add expense",
+  "titleLabel": "Title / description",
+  "titlePlaceholder": "e.g. Grocery shopping",
+  "categoryLabel": "Category",
+  "whoPaidLabel": "Who paid?",
+  "dateLabel": "Date",
+  "saving": "Saving…",
+  "submit": "Add expense",
+  "defaultTitle": "Expense",
+  "invalidAmount": "Enter a valid amount",
+  "selectPayer": "Choose who paid",
+  "selectFullOwedBy": "Choose who owes the full amount",
+  "offlineNotice": "No connection — the expense was saved locally and will sync automatically."
+}
--- a/frontend/src/i18n/locales/en/app.json
+++ b/frontend/src/i18n/locales/en/app.json
@@ -0,0 +1,3 @@
+{
+  "loading": "Loading…"
+}
--- a/frontend/src/i18n/locales/en/bottomNav.json
+++ b/frontend/src/i18n/locales/en/bottomNav.json
@@ -0,0 +1,7 @@
+{
+  "home": "Home",
+  "history": "History",
+  "settlements": "Settlements",
+  "stats": "Stats",
+  "addExpense": "Add expense"
+}
--- a/frontend/src/i18n/locales/en/charts.json
+++ b/frontend/src/i18n/locales/en/charts.json
@@ -0,0 +1,5 @@
+{
+  "noCategory": "No category",
+  "noExpensesThisMonth": "No expenses this month",
+  "noHistoricalData": "No historical data"
+}
--- a/frontend/src/i18n/locales/en/common.json
+++ b/frontend/src/i18n/locales/en/common.json
@@ -0,0 +1,22 @@
+{
+  "appName": "WhoWhat",
+  "save": "Save",
+  "saveChanges": "Save changes",
+  "saving": "Saving…",
+  "saved": "Saved",
+  "cancel": "Cancel",
+  "delete": "Delete",
+  "add": "Add",
+  "edit": "Edit",
+  "close": "Close",
+  "confirm": "Confirm",
+  "loading": "Loading…",
+  "yes": "Yes",
+  "no": "No",
+  "you": "You",
+  "copy": "Copy",
+  "copied": "Copied to clipboard!",
+  "copyFailed": "Couldn't copy automatically — select the field above and copy manually.",
+  "logout": "Log out",
+  "areYouSure": "Are you sure?"
+}
--- a/frontend/src/i18n/locales/en/confirmDialog.json
+++ b/frontend/src/i18n/locales/en/confirmDialog.json
@@ -0,0 +1,5 @@
+{
+  "defaultTitle": "Are you sure?",
+  "defaultConfirm": "Delete",
+  "defaultCancel": "Cancel"
+}
--- a/frontend/src/i18n/locales/en/dashboard.json
+++ b/frontend/src/i18n/locales/en/dashboard.json
@@ -0,0 +1,8 @@
+{
+  "settlementsTitle": "Settlements",
+  "allSettled": "You're all settled up!",
+  "settleUpButton": "Settle up",
+  "expensesByCategoryTitle": "Expenses by category (this month)",
+  "monthSummaryTitle": "Month summary",
+  "totalLabel": "Total"
+}
--- a/frontend/src/i18n/locales/en/errorBoundary.json
+++ b/frontend/src/i18n/locales/en/errorBoundary.json
@@ -0,0 +1,5 @@
+{
+  "title": "Something went wrong",
+  "subtitle": "An unexpected error occurred. Try refreshing the page.",
+  "refresh": "Refresh"
+}
--- a/frontend/src/i18n/locales/en/errors.json
+++ b/frontend/src/i18n/locales/en/errors.json
@@ -0,0 +1,40 @@
+{
+  "generic": "Something went wrong. Please try again.",
+  "already_a_member": "You're already a member of this household",
+  "already_settled": "You're already settled up",
+  "amount_must_be_positive": "The amount must be greater than zero",
+  "both_must_be_members": "Both people must be members of the household",
+  "category_name_required": "A category name is required",
+  "category_not_found": "Category not found",
+  "current_password_incorrect": "Current password is incorrect",
+  "email_already_registered": "An account with this email already exists",
+  "email_required": "Email is required",
+  "expense_not_found": "Expense not found",
+  "household_has_no_members": "This household has no members to split the expense between",
+  "household_not_found": "Household not found",
+  "household_not_selected": "No household selected",
+  "internal_server_error": "Internal server error",
+  "invalid_credentials": "Invalid email or password",
+  "invalid_or_expired_token": "Your session has expired — please log in again",
+  "invite_code_invalid_or_expired": "This invite code is invalid or has expired",
+  "invite_code_required": "An invite code is required",
+  "missing_bearer_token": "Authentication required",
+  "missing_expense_fields": "Amount, title, date, payer and split type are required",
+  "missing_login_fields": "Email and password are required",
+  "missing_password_fields": "Current and new password are required",
+  "missing_registration_fields": "Email, password and name are required",
+  "missing_reset_fields": "Token and password are required",
+  "missing_settlement_fields": "From, to and amount are required",
+  "name_required": "Name is required",
+  "not_a_household_member": "You're not a member of this household",
+  "password_too_short": "Password must be at least 6 characters",
+  "payer_and_recipient_must_differ": "The payer and recipient must be different people",
+  "payer_must_be_member": "The payer must be a member of the household",
+  "reset_link_invalid_or_expired": "This password reset link is invalid or has expired",
+  "settlement_not_found": "Settlement not found",
+  "share_assigned_to_non_member": "A share was assigned to someone outside the household",
+  "shares_must_sum_to_amount": "The shares must add up to the expense amount",
+  "shares_required": "Shares are required for this split type",
+  "unknown_split_type": "Unknown split type",
+  "user_not_found": "User not found"
+}
--- a/frontend/src/i18n/locales/en/expenseEditModal.json
+++ b/frontend/src/i18n/locales/en/expenseEditModal.json
@@ -0,0 +1,9 @@
+{
+  "title": "Edit expense",
+  "titleFieldLabel": "Title",
+  "categoryLabel": "Category",
+  "whoPaidLabel": "Who paid?",
+  "dateLabel": "Date",
+  "deleteConfirmTitle": "Delete this expense?",
+  "deleteConfirmMessage": "This can't be undone."
+}
--- a/frontend/src/i18n/locales/en/expenseListItem.json
+++ b/frontend/src/i18n/locales/en/expenseListItem.json
@@ -0,0 +1,3 @@
+{
+  "yourShare": "Your share"
+}
--- a/frontend/src/i18n/locales/en/forgotPassword.json
+++ b/frontend/src/i18n/locales/en/forgotPassword.json
@@ -0,0 +1,9 @@
+{
+  "title": "Reset password",
+  "sentMessage": "If an account with this email exists, we've sent a message with a password reset link.",
+  "subtitle": "Enter the email we should send the reset link to",
+  "emailPlaceholder": "Email",
+  "sending": "Sending…",
+  "submit": "Send link",
+  "backToLogin": "Back to login"
+}
--- a/frontend/src/i18n/locales/en/history.json
+++ b/frontend/src/i18n/locales/en/history.json
@@ -0,0 +1,7 @@
+{
+  "pageTitle": "History",
+  "allMonths": "All months",
+  "allCategories": "All categories",
+  "allPayers": "All payers",
+  "emptyState": "No entries match the filters"
+}
--- a/frontend/src/i18n/locales/en/installBanner.json
+++ b/frontend/src/i18n/locales/en/installBanner.json
@@ -0,0 +1,6 @@
+{
+  "installPrompt": "Install {{appName}} as an app on your phone",
+  "installIosPrompt": "Add {{appName}} to your home screen: tap Share, then “Add to Home Screen”",
+  "install": "Install",
+  "close": "Close"
+}
--- a/frontend/src/i18n/locales/en/joinInvite.json
+++ b/frontend/src/i18n/locales/en/joinInvite.json
@@ -0,0 +1,5 @@
+{
+  "title": "Joining household",
+  "waiting": "One moment…",
+  "goToApp": "Go to the app"
+}
--- a/frontend/src/i18n/locales/en/languageSwitcher.json
+++ b/frontend/src/i18n/locales/en/languageSwitcher.json
@@ -0,0 +1,3 @@
+{
+  "label": "Language"
+}
--- a/frontend/src/i18n/locales/en/login.json
+++ b/frontend/src/i18n/locales/en/login.json
@@ -0,0 +1,11 @@
+{
+  "subtitleInvite": "Log in to join the household you were invited to",
+  "subtitle": "Log in to manage shared expenses",
+  "emailPlaceholder": "Email",
+  "passwordPlaceholder": "Password",
+  "loggingIn": "Logging in…",
+  "submit": "Log in",
+  "forgotPassword": "Forgot your password?",
+  "noAccount": "Don't have an account?",
+  "register": "Sign up"
+}
--- a/frontend/src/i18n/locales/en/offlineBanner.json
+++ b/frontend/src/i18n/locales/en/offlineBanner.json
@@ -0,0 +1,4 @@
+{
+  "offline": "No connection — expenses are saved locally",
+  "syncing": "Syncing {{count}} expenses…"
+}
--- a/frontend/src/i18n/locales/en/onboarding.json
+++ b/frontend/src/i18n/locales/en/onboarding.json
@@ -0,0 +1,14 @@
+{
+  "back": "Back",
+  "title": "Welcome!",
+  "subtitle": "Create a new household or join your partner/roommate with a code",
+  "createTab": "Create",
+  "joinTab": "Join with code",
+  "defaultHouseholdName": "Our household",
+  "householdNamePlaceholder": "Household name",
+  "creating": "Creating…",
+  "createButton": "Create household",
+  "inviteCodePlaceholder": "Invite code",
+  "joining": "Joining…",
+  "joinButton": "Join"
+}
--- a/frontend/src/i18n/locales/en/passwordField.json
+++ b/frontend/src/i18n/locales/en/passwordField.json
@@ -0,0 +1,4 @@
+{
+  "show": "Show password",
+  "hide": "Hide password"
+}
--- a/frontend/src/i18n/locales/en/register.json
+++ b/frontend/src/i18n/locales/en/register.json
@@ -0,0 +1,11 @@
+{
+  "subtitleInvite": "Create an account to join the household you were invited to",
+  "subtitle": "Create an account to start splitting expenses",
+  "namePlaceholder": "Name",
+  "emailPlaceholder": "Email",
+  "passwordPlaceholder": "Password (min. 6 characters)",
+  "creating": "Creating account…",
+  "submit": "Sign up",
+  "haveAccount": "Already have an account?",
+  "login": "Log in"
+}
--- a/frontend/src/i18n/locales/en/resetPassword.json
+++ b/frontend/src/i18n/locales/en/resetPassword.json
@@ -0,0 +1,12 @@
+{
+  "passwordMismatch": "Passwords don't match",
+  "invalidLinkTitle": "Invalid link",
+  "invalidLinkMessage": "The password reset token is missing. Request a new link.",
+  "requestNewLink": "Reset password",
+  "title": "Set a new password",
+  "done": "Your password has been changed. Redirecting to login…",
+  "newPasswordPlaceholder": "New password (min. 6 characters)",
+  "confirmPasswordPlaceholder": "Repeat new password",
+  "saving": "Saving…",
+  "submit": "Set new password"
+}
--- a/frontend/src/i18n/locales/en/settings.json
+++ b/frontend/src/i18n/locales/en/settings.json
@@ -0,0 +1,81 @@
+{
+  "pageTitle": "Settings",
+  "install": {
+    "title": "App",
+    "installed": "The app is installed on this device",
+    "installButton": "Install app",
+    "iosPrompt": "Tap Share, then “Add to Home Screen”.",
+    "secureContextRequired": "Installing requires a secure connection — open the app at an address starting with {{https}}, not a local IP address.",
+    "notOfferedYet": "Your browser hasn't offered to install yet. Refresh the page after using the app for a bit, or use the browser menu (⋮) and choose “Install app” / “Add to Home Screen”."
+  },
+  "account": {
+    "title": "Account",
+    "nameLabel": "Name",
+    "emailLabel": "Email",
+    "deleteAccountButton": "Delete account",
+    "deleteConfirmTitle": "Delete your account?",
+    "deleteConfirmMessage": "This can't be undone.",
+    "deleteConfirmLabel": "Delete account"
+  },
+  "appearance": {
+    "title": "Appearance",
+    "light": "Light",
+    "dark": "Dark",
+    "system": "System"
+  },
+  "notifications": {
+    "title": "Notifications",
+    "emailLabel": "Email notifications",
+    "emailDesc": "You'll get an email when someone in the household adds a new expense"
+  },
+  "households": {
+    "title": "Your households",
+    "memberCountOne": "{{count}} member",
+    "memberCountOther": "{{count}} members",
+    "active": "Active",
+    "switch": "Switch",
+    "createOrJoin": "Create or join a household",
+    "activeNameLabel": "Active household name",
+    "currencyLabel": "Currency",
+    "membersHeading": "Members",
+    "youSuffix": "(You)",
+    "removeAria": "Remove",
+    "inviteAnother": "Invite another person with this code:",
+    "inviteLinkHint": "Or send a link that goes straight to sign-up and joins the household:",
+    "regenerateCode": "Generate new code",
+    "deleteHouseholdButton": "Delete household",
+    "leaveConfirmTitle": "Leave this household?",
+    "leaveConfirmMessage": "You'll need to join again with an invite code to come back.",
+    "leaveConfirmLabel": "Leave",
+    "removeMemberConfirmTitle": "Remove {{name}} from the household?",
+    "removeMemberConfirmMessage": "This person will lose access to the household (expense history is kept).",
+    "removeMemberConfirmLabel": "Remove",
+    "deleteHouseholdConfirmTitle": "Delete household “{{name}}”?",
+    "deleteHouseholdConfirmMessage": "This deletes the entire expense history for all members. This can't be undone.",
+    "deleteHouseholdConfirmLabel": "Delete household"
+  },
+  "security": {
+    "title": "Security",
+    "currentPasswordLabel": "Current password",
+    "newPasswordLabel": "New password",
+    "confirmPasswordLabel": "Repeat new password",
+    "passwordMismatch": "New passwords don't match",
+    "passwordChanged": "Password changed",
+    "changePasswordButton": "Change password"
+  },
+  "categories": {
+    "title": "Categories",
+    "iconLabel": "Icon",
+    "nameLabel": "Category name",
+    "namePlaceholder": "e.g. Pets",
+    "addButton": "Add category",
+    "deleteAria": "Delete",
+    "deleteConfirmTitle": "Delete category “{{name}}”?",
+    "deleteConfirmMessage": "Existing expenses keep their history but lose their assigned category.",
+    "deleteConfirmLabel": "Delete"
+  },
+  "data": {
+    "title": "Data",
+    "downloadCsv": "Download CSV"
+  }
+}
--- a/frontend/src/i18n/locales/en/settingsButton.json
+++ b/frontend/src/i18n/locales/en/settingsButton.json
@@ -0,0 +1,3 @@
+{
+  "settings": "Settings"
+}
--- a/frontend/src/i18n/locales/en/settlementEditModal.json
+++ b/frontend/src/i18n/locales/en/settlementEditModal.json
@@ -0,0 +1,7 @@
+{
+  "title": "Edit settlement",
+  "whoPaidLabel": "Who paid?",
+  "toWhomLabel": "To whom?",
+  "deleteConfirmTitle": "Delete this settlement?",
+  "deleteConfirmMessage": "The balance will be recalculated as if this payment never happened."
+}
--- a/frontend/src/i18n/locales/en/settlementListItem.json
+++ b/frontend/src/i18n/locales/en/settlementListItem.json
@@ -0,0 +1,3 @@
+{
+  "settlement": "Settlement"
+}
--- a/frontend/src/i18n/locales/en/settlements.json
+++ b/frontend/src/i18n/locales/en/settlements.json
@@ -0,0 +1,17 @@
+{
+  "pageTitle": "Settlements",
+  "settlementsTitle": "Settlements",
+  "allSettled": "You're all settled up!",
+  "suggestedTitle": "Suggested settlement (tap to fill the form below)",
+  "settleAllButton": "Settle everything automatically",
+  "settling": "Settling…",
+  "recordPaymentTitle": "Record a payment",
+  "whoPaysLabel": "Who's paying?",
+  "toWhomLabel": "To whom?",
+  "saveSettlementButton": "Save settlement",
+  "saving": "Saving…",
+  "savedNotice": "Payment saved!",
+  "historyTitle": "Settlement history",
+  "emptyState": "No settlements recorded yet",
+  "selectBothAndAmount": "Select both people and enter an amount greater than zero"
+}
--- a/frontend/src/i18n/locales/en/splitSelector.json
+++ b/frontend/src/i18n/locales/en/splitSelector.json
@@ -0,0 +1,9 @@
+{
+  "label": "How do we split this?",
+  "equal": "Equally (50/50)",
+  "exact": "Exact amounts",
+  "full": "Full amount to one person",
+  "autoCalculated": "(calculated automatically)",
+  "overAllocated": "The shares exceed the expense amount",
+  "owesFullAmount": "{{name}} owes the full amount"
+}
--- a/frontend/src/i18n/locales/en/stats.json
+++ b/frontend/src/i18n/locales/en/stats.json
@@ -0,0 +1,6 @@
+{
+  "pageTitle": "Stats",
+  "monthOverMonthTitle": "Month-over-month expenses",
+  "comparisonTitle": "Who's spending more (this month)",
+  "categoriesTitle": "Categories, highest first"
+}
--- a/frontend/src/i18n/locales/pl/addExpense.json
+++ b/frontend/src/i18n/locales/pl/addExpense.json
@@ -0,0 +1,15 @@
+{
+  "pageTitle": "Dodaj wydatek",
+  "titleLabel": "Tytuł / opis",
+  "titlePlaceholder": "np. Zakupy Biedronka",
+  "categoryLabel": "Kategoria",
+  "whoPaidLabel": "Kto płacił?",
+  "dateLabel": "Data",
+  "saving": "Zapisywanie…",
+  "submit": "Dodaj wydatek",
+  "defaultTitle": "Wydatek",
+  "invalidAmount": "Podaj poprawną kwotę",
+  "selectPayer": "Wybierz kto płacił",
+  "selectFullOwedBy": "Wybierz kto jest winien całość",
+  "offlineNotice": "Brak sieci — wydatek zapisano lokalnie i zsynchronizuje się automatycznie."
+}
--- a/frontend/src/i18n/locales/pl/app.json
+++ b/frontend/src/i18n/locales/pl/app.json
@@ -0,0 +1,3 @@
+{
+  "loading": "Ładowanie…"
+}
--- a/frontend/src/i18n/locales/pl/bottomNav.json
+++ b/frontend/src/i18n/locales/pl/bottomNav.json
@@ -0,0 +1,7 @@
+{
+  "home": "Start",
+  "history": "Historia",
+  "settlements": "Rozliczenia",
+  "stats": "Statystyki",
+  "addExpense": "Dodaj wydatek"
+}
--- a/frontend/src/i18n/locales/pl/charts.json
+++ b/frontend/src/i18n/locales/pl/charts.json
@@ -0,0 +1,5 @@
+{
+  "noCategory": "Bez kategorii",
+  "noExpensesThisMonth": "Brak wydatków w tym miesiącu",
+  "noHistoricalData": "Brak danych historycznych"
+}
--- a/frontend/src/i18n/locales/pl/common.json
+++ b/frontend/src/i18n/locales/pl/common.json
@@ -0,0 +1,22 @@
+{
+  "appName": "WhoWhat",
+  "save": "Zapisz",
+  "saveChanges": "Zapisz zmiany",
+  "saving": "Zapisywanie…",
+  "saved": "Zapisano",
+  "cancel": "Anuluj",
+  "delete": "Usuń",
+  "add": "Dodaj",
+  "edit": "Edytuj",
+  "close": "Zamknij",
+  "confirm": "Potwierdź",
+  "loading": "Ładowanie…",
+  "yes": "Tak",
+  "no": "Nie",
+  "you": "Ty",
+  "copy": "Kopiuj",
+  "copied": "Skopiowano do schowka!",
+  "copyFailed": "Nie udało się skopiować automatycznie — zaznacz pole powyżej i skopiuj ręcznie.",
+  "logout": "Wyloguj się",
+  "areYouSure": "Czy na pewno?"
+}
--- a/frontend/src/i18n/locales/pl/confirmDialog.json
+++ b/frontend/src/i18n/locales/pl/confirmDialog.json
@@ -0,0 +1,5 @@
+{
+  "defaultTitle": "Czy na pewno?",
+  "defaultConfirm": "Usuń",
+  "defaultCancel": "Anuluj"
+}
--- a/frontend/src/i18n/locales/pl/dashboard.json
+++ b/frontend/src/i18n/locales/pl/dashboard.json
@@ -0,0 +1,8 @@
+{
+  "settlementsTitle": "Rozliczenia",
+  "allSettled": "Jesteście na czysto!",
+  "settleUpButton": "Rozlicz się",
+  "expensesByCategoryTitle": "Wydatki wg kategorii (ten miesiąc)",
+  "monthSummaryTitle": "Podsumowanie miesiąca",
+  "totalLabel": "Suma"
+}
--- a/frontend/src/i18n/locales/pl/errorBoundary.json
+++ b/frontend/src/i18n/locales/pl/errorBoundary.json
@@ -0,0 +1,5 @@
+{
+  "title": "Coś poszło nie tak",
+  "subtitle": "Wystąpił nieoczekiwany błąd. Spróbuj odświeżyć stronę.",
+  "refresh": "Odśwież"
+}
--- a/frontend/src/i18n/locales/pl/errors.json
+++ b/frontend/src/i18n/locales/pl/errors.json
@@ -0,0 +1,40 @@
+{
+  "generic": "Coś poszło nie tak. Spróbuj ponownie.",
+  "already_a_member": "Jesteś już członkiem tego gospodarstwa",
+  "already_settled": "Jesteście już rozliczeni",
+  "amount_must_be_positive": "Kwota musi być większa od zera",
+  "both_must_be_members": "Obie osoby muszą być członkami gospodarstwa",
+  "category_name_required": "Nazwa kategorii jest wymagana",
+  "category_not_found": "Kategoria nie znaleziona",
+  "current_password_incorrect": "Bieżące hasło jest nieprawidłowe",
+  "email_already_registered": "Konto z tym adresem e-mail już istnieje",
+  "email_required": "E-mail jest wymagany",
+  "expense_not_found": "Wydatek nie znaleziony",
+  "household_has_no_members": "Gospodarstwo domowe nie ma członków do podziału wydatku",
+  "household_not_found": "Gospodarstwo nie znalezione",
+  "household_not_selected": "Nie wybrano gospodarstwa",
+  "internal_server_error": "Wewnętrzny błąd serwera",
+  "invalid_credentials": "Nieprawidłowy e-mail lub hasło",
+  "invalid_or_expired_token": "Twoja sesja wygasła — zaloguj się ponownie",
+  "invite_code_invalid_or_expired": "Kod zaproszenia jest nieprawidłowy lub wygasł",
+  "invite_code_required": "Kod zaproszenia jest wymagany",
+  "missing_bearer_token": "Wymagane jest zalogowanie",
+  "missing_expense_fields": "Kwota, tytuł, data, płacący i typ podziału są wymagane",
+  "missing_login_fields": "E-mail i hasło są wymagane",
+  "missing_password_fields": "Bieżące i nowe hasło są wymagane",
+  "missing_registration_fields": "E-mail, hasło i imię są wymagane",
+  "missing_reset_fields": "Token i hasło są wymagane",
+  "missing_settlement_fields": "Nadawca, odbiorca i kwota są wymagane",
+  "name_required": "Imię jest wymagane",
+  "not_a_household_member": "Nie jesteś członkiem tego gospodarstwa",
+  "password_too_short": "Hasło musi mieć co najmniej 6 znaków",
+  "payer_and_recipient_must_differ": "Płacący i odbiorca muszą być różnymi osobami",
+  "payer_must_be_member": "Płacący musi być członkiem gospodarstwa domowego",
+  "reset_link_invalid_or_expired": "Link do resetu hasła jest nieprawidłowy lub wygasł",
+  "settlement_not_found": "Rozliczenie nie znalezione",
+  "share_assigned_to_non_member": "Udział przypisano osobie spoza gospodarstwa domowego",
+  "shares_must_sum_to_amount": "Suma udziałów musi być równa kwocie wydatku",
+  "shares_required": "Udziały są wymagane dla wybranego typu podziału",
+  "unknown_split_type": "Nieznany typ podziału",
+  "user_not_found": "Użytkownik nie znaleziony"
+}
--- a/frontend/src/i18n/locales/pl/expenseEditModal.json
+++ b/frontend/src/i18n/locales/pl/expenseEditModal.json
@@ -0,0 +1,9 @@
+{
+  "title": "Edytuj wydatek",
+  "titleFieldLabel": "Tytuł",
+  "categoryLabel": "Kategoria",
+  "whoPaidLabel": "Kto płacił?",
+  "dateLabel": "Data",
+  "deleteConfirmTitle": "Usunąć ten wydatek?",
+  "deleteConfirmMessage": "Tej operacji nie da się cofnąć."
+}
--- a/frontend/src/i18n/locales/pl/expenseListItem.json
+++ b/frontend/src/i18n/locales/pl/expenseListItem.json
@@ -0,0 +1,3 @@
+{
+  "yourShare": "Twoja część"
+}
--- a/frontend/src/i18n/locales/pl/forgotPassword.json
+++ b/frontend/src/i18n/locales/pl/forgotPassword.json
@@ -0,0 +1,9 @@
+{
+  "title": "Przypomnij hasło",
+  "sentMessage": "Jeśli konto z tym adresem e-mail istnieje, wysłaliśmy wiadomość z linkiem do resetu hasła.",
+  "subtitle": "Podaj e-mail, na który wyślemy link do zresetowania hasła",
+  "emailPlaceholder": "E-mail",
+  "sending": "Wysyłanie…",
+  "submit": "Wyślij link",
+  "backToLogin": "Powrót do logowania"
+}
--- a/frontend/src/i18n/locales/pl/history.json
+++ b/frontend/src/i18n/locales/pl/history.json
@@ -0,0 +1,7 @@
+{
+  "pageTitle": "Historia",
+  "allMonths": "Wszystkie miesiące",
+  "allCategories": "Wszystkie kategorie",
+  "allPayers": "Wszyscy płacący",
+  "emptyState": "Brak wpisów spełniających filtry"
+}
--- a/frontend/src/i18n/locales/pl/installBanner.json
+++ b/frontend/src/i18n/locales/pl/installBanner.json
@@ -0,0 +1,6 @@
+{
+  "installPrompt": "Zainstaluj {{appName}} jako aplikację na telefonie",
+  "installIosPrompt": "Dodaj {{appName}} do ekranu głównego: dotknij Udostępnij, a potem „Dodaj do ekranu początkowego”",
+  "install": "Zainstaluj",
+  "close": "Zamknij"
+}
--- a/frontend/src/i18n/locales/pl/joinInvite.json
+++ b/frontend/src/i18n/locales/pl/joinInvite.json
@@ -0,0 +1,5 @@
+{
+  "title": "Dołączanie do gospodarstwa",
+  "waiting": "Chwileczkę…",
+  "goToApp": "Przejdź do aplikacji"
+}
--- a/frontend/src/i18n/locales/pl/languageSwitcher.json
+++ b/frontend/src/i18n/locales/pl/languageSwitcher.json
@@ -0,0 +1,3 @@
+{
+  "label": "Język"
+}
--- a/frontend/src/i18n/locales/pl/login.json
+++ b/frontend/src/i18n/locales/pl/login.json
@@ -0,0 +1,11 @@
+{
+  "subtitleInvite": "Zaloguj się, aby dołączyć do zaproszonego gospodarstwa domowego",
+  "subtitle": "Zaloguj się, by zarządzać wspólnymi wydatkami",
+  "emailPlaceholder": "E-mail",
+  "passwordPlaceholder": "Hasło",
+  "loggingIn": "Logowanie…",
+  "submit": "Zaloguj się",
+  "forgotPassword": "Zapomniałeś hasła?",
+  "noAccount": "Nie masz konta?",
+  "register": "Zarejestruj się"
+}
--- a/frontend/src/i18n/locales/pl/offlineBanner.json
+++ b/frontend/src/i18n/locales/pl/offlineBanner.json
@@ -0,0 +1,4 @@
+{
+  "offline": "Brak połączenia — wydatki zapisują się lokalnie",
+  "syncing": "Synchronizowanie {{count}} wydatków…"
+}
--- a/frontend/src/i18n/locales/pl/onboarding.json
+++ b/frontend/src/i18n/locales/pl/onboarding.json
@@ -0,0 +1,14 @@
+{
+  "back": "Cofnij",
+  "title": "Witaj!",
+  "subtitle": "Załóż nowe gospodarstwo domowe albo dołącz do partnera/partnerki kodem",
+  "createTab": "Utwórz",
+  "joinTab": "Dołącz kodem",
+  "defaultHouseholdName": "Nasze gospodarstwo",
+  "householdNamePlaceholder": "Nazwa gospodarstwa",
+  "creating": "Tworzenie…",
+  "createButton": "Utwórz gospodarstwo",
+  "inviteCodePlaceholder": "Kod zaproszenia",
+  "joining": "Dołączanie…",
+  "joinButton": "Dołącz"
+}
--- a/frontend/src/i18n/locales/pl/passwordField.json
+++ b/frontend/src/i18n/locales/pl/passwordField.json
@@ -0,0 +1,4 @@
+{
+  "show": "Pokaż hasło",
+  "hide": "Ukryj hasło"
+}
--- a/frontend/src/i18n/locales/pl/register.json
+++ b/frontend/src/i18n/locales/pl/register.json
@@ -0,0 +1,11 @@
+{
+  "subtitleInvite": "Załóż konto, aby dołączyć do zaproszonego gospodarstwa domowego",
+  "subtitle": "Załóż konto, by zacząć dzielić wydatki",
+  "namePlaceholder": "Imię",
+  "emailPlaceholder": "E-mail",
+  "passwordPlaceholder": "Hasło (min. 6 znaków)",
+  "creating": "Tworzenie konta…",
+  "submit": "Zarejestruj się",
+  "haveAccount": "Masz już konto?",
+  "login": "Zaloguj się"
+}
--- a/frontend/src/i18n/locales/pl/resetPassword.json
+++ b/frontend/src/i18n/locales/pl/resetPassword.json
@@ -0,0 +1,12 @@
+{
+  "passwordMismatch": "Hasła nie są takie same",
+  "invalidLinkTitle": "Nieprawidłowy link",
+  "invalidLinkMessage": "Brakuje tokenu resetu hasła. Poproś o nowy link.",
+  "requestNewLink": "Przypomnij hasło",
+  "title": "Ustaw nowe hasło",
+  "done": "Hasło zostało zmienione. Przenoszenie do logowania…",
+  "newPasswordPlaceholder": "Nowe hasło (min. 6 znaków)",
+  "confirmPasswordPlaceholder": "Powtórz nowe hasło",
+  "saving": "Zapisywanie…",
+  "submit": "Ustaw nowe hasło"
+}
--- a/frontend/src/i18n/locales/pl/settings.json
+++ b/frontend/src/i18n/locales/pl/settings.json
@@ -0,0 +1,81 @@
+{
+  "pageTitle": "Ustawienia",
+  "install": {
+    "title": "Aplikacja",
+    "installed": "Aplikacja jest zainstalowana na tym urządzeniu",
+    "installButton": "Zainstaluj aplikację",
+    "iosPrompt": "Dotknij Udostępnij, a potem „Dodaj do ekranu początkowego”.",
+    "secureContextRequired": "Instalacja wymaga bezpiecznego połączenia — otwórz aplikację pod adresem zaczynającym się od {{https}}, a nie przez lokalny adres IP.",
+    "notOfferedYet": "Przeglądarka jeszcze nie zaproponowała instalacji. Odśwież stronę po chwili korzystania z aplikacji albo użyj menu przeglądarki (⋮) i wybierz „Zainstaluj aplikację” / „Dodaj do ekranu głównego”."
+  },
+  "account": {
+    "title": "Konto",
+    "nameLabel": "Imię",
+    "emailLabel": "E-mail",
+    "deleteAccountButton": "Usuń konto",
+    "deleteConfirmTitle": "Usunąć swoje konto?",
+    "deleteConfirmMessage": "Tej operacji nie da się cofnąć.",
+    "deleteConfirmLabel": "Usuń konto"
+  },
+  "appearance": {
+    "title": "Wygląd",
+    "light": "Jasny",
+    "dark": "Ciemny",
+    "system": "Systemowy"
+  },
+  "notifications": {
+    "title": "Powiadomienia",
+    "emailLabel": "Powiadomienia mailowe",
+    "emailDesc": "Dostaniesz e-mail, gdy ktoś w gospodarstwie doda nowy wydatek"
+  },
+  "households": {
+    "title": "Twoje gospodarstwa",
+    "memberCountOne": "{{count}} osoba",
+    "memberCountOther": "{{count}} osoby/osób",
+    "active": "Aktywne",
+    "switch": "Przełącz",
+    "createOrJoin": "Utwórz lub dołącz do gospodarstwa",
+    "activeNameLabel": "Nazwa aktywnego gospodarstwa",
+    "currencyLabel": "Waluta",
+    "membersHeading": "Członkowie",
+    "youSuffix": "(Ty)",
+    "removeAria": "Usuń",
+    "inviteAnother": "Zaproś kolejną osobę tym kodem:",
+    "inviteLinkHint": "Albo wyślij link, który od razu przeniesie do rejestracji i dołączy do gospodarstwa:",
+    "regenerateCode": "Wygeneruj nowy kod",
+    "deleteHouseholdButton": "Usuń gospodarstwo",
+    "leaveConfirmTitle": "Opuścić to gospodarstwo?",
+    "leaveConfirmMessage": "Będziesz musiał(a) dołączyć ponownie kodem zaproszenia, żeby wrócić.",
+    "leaveConfirmLabel": "Opuść",
+    "removeMemberConfirmTitle": "Usunąć {{name}} z gospodarstwa?",
+    "removeMemberConfirmMessage": "Ta osoba straci dostęp do gospodarstwa (historia wydatków zostanie zachowana).",
+    "removeMemberConfirmLabel": "Usuń",
+    "deleteHouseholdConfirmTitle": "Usunąć gospodarstwo „{{name}}”?",
+    "deleteHouseholdConfirmMessage": "Usunie to całą historię wydatków wszystkich członków. Tej operacji nie da się cofnąć.",
+    "deleteHouseholdConfirmLabel": "Usuń gospodarstwo"
+  },
+  "security": {
+    "title": "Bezpieczeństwo",
+    "currentPasswordLabel": "Bieżące hasło",
+    "newPasswordLabel": "Nowe hasło",
+    "confirmPasswordLabel": "Powtórz nowe hasło",
+    "passwordMismatch": "Nowe hasła nie są takie same",
+    "passwordChanged": "Hasło zostało zmienione",
+    "changePasswordButton": "Zmień hasło"
+  },
+  "categories": {
+    "title": "Kategorie",
+    "iconLabel": "Ikona",
+    "nameLabel": "Nazwa kategorii",
+    "namePlaceholder": "np. Zwierzęta",
+    "addButton": "Dodaj kategorię",
+    "deleteAria": "Usuń",
+    "deleteConfirmTitle": "Usunąć kategorię „{{name}}”?",
+    "deleteConfirmMessage": "Istniejące wydatki zachowają swoją historię, ale stracą przypisaną kategorię.",
+    "deleteConfirmLabel": "Usuń"
+  },
+  "data": {
+    "title": "Dane",
+    "downloadCsv": "Pobierz CSV"
+  }
+}
--- a/frontend/src/i18n/locales/pl/settingsButton.json
+++ b/frontend/src/i18n/locales/pl/settingsButton.json
@@ -0,0 +1,3 @@
+{
+  "settings": "Ustawienia"
+}
--- a/frontend/src/i18n/locales/pl/settlementEditModal.json
+++ b/frontend/src/i18n/locales/pl/settlementEditModal.json
@@ -0,0 +1,7 @@
+{
+  "title": "Edytuj rozliczenie",
+  "whoPaidLabel": "Kto płacił?",
+  "toWhomLabel": "Komu?",
+  "deleteConfirmTitle": "Usunąć to rozliczenie?",
+  "deleteConfirmMessage": "Saldo zostanie przeliczone tak, jakby ta płatność nigdy nie miała miejsca."
+}
--- a/frontend/src/i18n/locales/pl/settlementListItem.json
+++ b/frontend/src/i18n/locales/pl/settlementListItem.json
@@ -0,0 +1,3 @@
+{
+  "settlement": "Rozliczenie"
+}
--- a/frontend/src/i18n/locales/pl/settlements.json
+++ b/frontend/src/i18n/locales/pl/settlements.json
@@ -0,0 +1,17 @@
+{
+  "pageTitle": "Rozliczenia",
+  "settlementsTitle": "Rozliczenia",
+  "allSettled": "Jesteście na czysto!",
+  "suggestedTitle": "Sugerowane rozliczenie (dotknij, aby wypełnić formularz poniżej)",
+  "settleAllButton": "Rozlicz wszystko automatycznie",
+  "settling": "Rozliczanie…",
+  "recordPaymentTitle": "Zapisz płatność",
+  "whoPaysLabel": "Kto płaci?",
+  "toWhomLabel": "Komu?",
+  "saveSettlementButton": "Zapisz rozliczenie",
+  "saving": "Zapisywanie…",
+  "savedNotice": "Zapisano płatność!",
+  "historyTitle": "Historia rozliczeń",
+  "emptyState": "Brak zapisanych rozliczeń",
+  "selectBothAndAmount": "Wybierz obie osoby i podaj kwotę większą od zera"
+}
--- a/frontend/src/i18n/locales/pl/splitSelector.json
+++ b/frontend/src/i18n/locales/pl/splitSelector.json
@@ -0,0 +1,9 @@
+{
+  "label": "Jak dzielimy wydatek?",
+  "equal": "Po równo (50/50)",
+  "exact": "Dokładny podział",
+  "full": "Całość na jedną osobę",
+  "autoCalculated": "(wyliczane automatycznie)",
+  "overAllocated": "Suma udziałów przekracza kwotę wydatku",
+  "owesFullAmount": "{{name}} winien(-na) całość"
+}
--- a/frontend/src/i18n/locales/pl/stats.json
+++ b/frontend/src/i18n/locales/pl/stats.json
@@ -0,0 +1,6 @@
+{
+  "pageTitle": "Statystyki",
+  "monthOverMonthTitle": "Wydatki miesiąc do miesiąca",
+  "comparisonTitle": "Kto więcej konsumuje (ten miesiąc)",
+  "categoriesTitle": "Kategorie od najdroższej"
+}
--- a/Show More
+++ b/Show More