docs(34-04): complete language picker and i18n sync plan summary

- Language picker in settings using pill-toggle pattern (English/Deutsch)
- i18n sync with DB setting on load via useEffect in RootLayout
- Both tasks verified complete at commit 46715cc

.planning/phases/34-i18n-foundation/34-04-SUMMARY.md

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+---
+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*