# Shared i18n architecture (frontend + backend)

## Scope
Issue #175 requires one shared i18n contract for MVP host/player flows across frontend and backend.

## Source of truth
- Catalog: `shared/i18n/lobby.json`
- Locales:
  - supported: `en`, `da`
  - default/fallback: `en`

Both Angular (`frontend/angular/src/app/lobby-i18n.ts`) and Django (`lobby/i18n.py`) read from this catalog.

## Key naming convention
- Domain-first namespaces:
  - `frontend.ui.common.*`
  - `frontend.ui.host.*`
  - `frontend.ui.player.*`
  - `frontend.errors.*`
  - `backend.errors.*`
  - `backend.error_codes.*`
- UI lookup keys in Angular are shortened aliases mapped under `frontend.ui`, e.g.:
  - `host.start_round`
  - `player.submit_guess`
  - `common.session_code`
- Backend API errors return stable code + localized message:
  - `error_code` = machine-stable key from `backend.error_codes`
  - `error` = localized message from `backend.errors`
  - `locale` = resolved request locale

## Fallback model (robust)
1. Resolve requested locale (`Accept-Language` on backend, user/browser preference on frontend).
2. If locale unsupported -> use default `en`.
3. If key missing -> return key and log warning.
4. If locale translation missing for key -> fallback to `en` translation.

## Audio-routing policy
- Catalog capability: `frontend.capabilities.client_has_no_audio_output = true`
- Host/player clients expose this as a read-only capability flag.
- Policy: phone clients must not play audio directly; only primary/host output is allowed.

## Verification
- Backend tests: `lobby/tests.py` i18n coverage for locale selection + fallback + error-code/message matrix.
- Frontend smoke/e2e-level unit coverage:
  - `frontend/angular/src/app/lobby-i18n.spec.ts`
  - `frontend/angular/src/app/i18n-mvp-flow-smoke.spec.ts`
  - `frontend/angular/src/app/features/host/host-shell.component.spec.ts`
  - `frontend/angular/src/app/features/player/player-shell.component.spec.ts`