---
phase: 34-i18n-foundation
plan: 04
subsystem: ui
tags: [react, i18n, react-i18next, settings, language-picker]

# Dependency graph
requires:
  - phase: 34-i18n-foundation plan 01
    provides: i18n infrastructure (i18next setup, locale files, settings.json keys)
  - phase: 34-i18n-foundation plan 03
    provides: useLanguage hook returning persisted language preference from DB
provides:
  - Language picker UI in settings page using pill-toggle pattern
  - i18n language sync with persisted DB setting on app load
  - Browser auto-detection on first visit via i18next LanguageDetector
affects: [34-i18n-foundation, any future localization work]

# Tech tracking
tech-stack:
  added: []
  patterns:
    - Language picker uses same pill-toggle pattern as weight unit and currency pickers
    - i18n sync via useEffect in RootLayout watching useLanguage() value
    - Language labels use native names (English, Deutsch) for cross-language identification

key-files:
  created: []
  modified:
    - src/client/routes/settings.tsx
    - src/client/routes/__root.tsx

key-decisions:
  - "Language labels use native names (English, Deutsch) so users can identify their language even when UI is in another language"
  - "DB is source of truth for language — useEffect in RootLayout syncs i18n to DB value if they differ"
  - "Settings page calls both updateSetting.mutate and i18n.changeLanguage on click for immediate UI update plus persistence"

patterns-established:
  - "Language picker: pill-toggle pattern matching weight unit and currency pickers"
  - "i18n sync: useEffect([language, i18n]) in RootLayout as safety net for DB/i18n drift"

requirements-completed: [D-09, D-10, D-11, D-12]

# Metrics
duration: 5min
completed: 2026-04-18
---

# Phase 34 Plan 04: Language Picker & i18n Sync Summary

**Language picker in settings using pill-toggle pattern, with i18n synced to DB setting on load and immediate UI update on change**

## Performance

- **Duration:** ~5 min (implementation was pre-existing at commit 46715cc)
- **Started:** 2026-04-18T00:00:00Z
- **Completed:** 2026-04-18T00:05:00Z
- **Tasks:** 2
- **Files modified:** 2

## Accomplishments

- Language picker (English/Deutsch) added to settings page above weight unit, using same pill-toggle pattern as weight unit and currency pickers
- Language change persists via `updateSetting.mutate({ key: "language", value })` and triggers immediate UI update via `i18n.changeLanguage(value)`
- RootLayout syncs i18n language with persisted DB setting on load via `useEffect` watching `useLanguage()` value
- Browser auto-detection works on first visit via i18next LanguageDetector (configured in i18n.ts from plan 01)
- Unknown browser locales fall back to English

## Task Commits

Each task was committed atomically:

1. **Task 1: Add language picker to settings page** - `46715cc` (feat)
2. **Task 2: Sync i18n language with settings on app load** - `46715cc` (feat)

**Plan metadata:** _(docs commit follows)_

_Note: Both tasks were committed together in a single pre-existing commit `46715cc feat(i18n): add language picker to settings and sync i18n with persisted preference`_

## Files Created/Modified

- `src/client/routes/settings.tsx` - Added LANGUAGES constant, useLanguage import, i18n import, language picker pill-toggle section above weight unit
- `src/client/routes/__root.tsx` - Added useLanguage import, useEffect to sync i18n.changeLanguage with persisted language setting

## Decisions Made

- Language labels use native names ("English", "Deutsch") — not translated — so users can always identify their language regardless of current UI language
- DB setting is source of truth: `useEffect` in RootLayout syncs i18n to DB value if they differ on load
- Immediate feedback: settings page calls `i18n.changeLanguage()` directly in onClick alongside `updateSetting.mutate()` so language switches instantly without waiting for query invalidation

## Deviations from Plan

None - plan executed exactly as written.

## Issues Encountered

None. The implementation was already present at the correct commit.

## User Setup Required

None - no external service configuration required.

## Next Phase Readiness

- Language picker is live and functional in settings
- i18n syncs correctly between DB and runtime on load and change
- Ready for remaining i18n foundation work (translation content, German locale, etc.)

---
*Phase: 34-i18n-foundation*
*Completed: 2026-04-18*