feat: Implement frontend localization with i18n and measurement units
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Implemented comprehensive frontend localization infrastructure supporting
5 languages (English, Spanish, French, Portuguese, Chinese) with measurement
unit preferences (Metric/Imperial). This lays the foundation for international
user support.

**Core Infrastructure:**
- Installed i18next, react-i18next, i18next-browser-languagedetector
- Created I18nProvider component integrated into app layout
- Configured i18next with language detection and localStorage persistence
- Created 35 translation files (5 languages × 7 namespaces)

**Translation Namespaces:**
- common: App-wide UI elements, navigation, actions
- tracking: Activity tracking (feeding, sleep, diaper, milestones)
- ai: AI assistant chat interface
- auth: Authentication flows (login, signup, password reset)
- settings: Settings and preferences
- onboarding: Onboarding flow
- errors: Error messages and validation

**Custom Hooks:**
- useTranslation: Type-safe translation wrapper
- useLocale: Language and measurement system management
- useFormatting: Date, time, number, and unit formatting

**Measurement Unit Support:**
- Created unit conversion utilities (weight, height, temperature, volume)
- Metric: kg, cm, °C, ml
- Imperial: lb, in, °F, oz
- Bidirectional conversion functions

**UI Components:**
- LanguageSelector: Dropdown to change app language
- MeasurementUnitSelector: Toggle between Metric/Imperial
- Integrated both into Settings page Preferences section

**Next Steps (Remaining):**
- Add measurement preferences to backend user schema
- Create onboarding flow with language/measurement selection
- Apply translations to existing components (dashboard, tracking forms)
- Implement multi-language AI responses
- Add professional translations (currently using basic translations)

**File Highlights:**
- lib/i18n/config.ts: i18next configuration
- hooks/useFormatting.ts: Formatting utilities with locale support
- lib/utils/unitConversion.ts: Unit conversion logic
- components/settings/*Selector.tsx: Language and measurement selectors
- locales/*/: Translation files for 5 languages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
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# Localization Implementation Plan
**Created**: October 3, 2025
**Priority**: HIGH (Pre-Launch)
**Estimated Duration**: 2-3 days
## Overview
Implement comprehensive internationalization (i18n) support for the Maternal App with 5 languages and measurement unit preferences.
## Supported Languages
1. **English (en-US)** - Primary/Default
2. **Spanish (es-ES)**
3. **French (fr-FR)**
4. **Portuguese (pt-BR)**
5. **Simplified Chinese (zh-CN)**
## Current Status
### ✅ Already Completed
- Backend multilanguage support for AI responses
- AI safety responses in 5 languages
- MultiLanguageService with language detection
### ❌ To Be Implemented
- Frontend i18next framework
- Translation files for all UI strings
- Language selector in onboarding
- Language selector in settings
- Date/time localization
- Measurement unit preferences (Metric/Imperial)
- Number formatting per locale
---
## Phase 1: Framework Setup
### 1.1 Install Dependencies
**Files**: `package.json`
```bash
npm install i18next react-i18next i18next-browser-languagedetector
npm install date-fns # Already installed, confirm locales
```
**Packages**:
- `i18next` - Core i18n framework
- `react-i18next` - React bindings
- `i18next-browser-languagedetector` - Auto-detect user language
### 1.2 Create i18n Configuration
**File**: `lib/i18n/config.ts` (NEW)
- Initialize i18next
- Configure language detector
- Set up fallback language (en-US)
- Configure interpolation
- Load translation resources
### 1.3 Create i18n Provider
**File**: `components/providers/I18nProvider.tsx` (NEW)
- Wrap app with I18nextProvider
- Initialize i18n on mount
- Handle language loading states
### 1.4 Update Root Layout
**File**: `app/layout.tsx` (MODIFY)
- Add I18nProvider to provider stack
- Set html lang attribute dynamically
---
## Phase 2: Translation Files Structure
### 2.1 Directory Structure
```
locales/
├── en/
│ ├── common.json # Common UI strings (buttons, labels, etc.)
│ ├── auth.json # Authentication pages
│ ├── dashboard.json # Dashboard/home page
│ ├── tracking.json # Activity tracking
│ ├── children.json # Child management
│ ├── family.json # Family management
│ ├── ai.json # AI assistant
│ ├── analytics.json # Analytics/insights
│ ├── settings.json # Settings page
│ ├── onboarding.json # Onboarding flow
│ ├── errors.json # Error messages
│ └── validation.json # Form validation messages
├── es/
│ └── [same structure]
├── fr/
│ └── [same structure]
├── pt/
│ └── [same structure]
└── zh/
└── [same structure]
```
### 2.2 Translation Keys Structure
**Example - common.json**:
```json
{
"nav": {
"dashboard": "Dashboard",
"track": "Track",
"children": "Children",
"family": "Family",
"ai": "AI Assistant",
"analytics": "Analytics",
"settings": "Settings"
},
"buttons": {
"save": "Save",
"cancel": "Cancel",
"delete": "Delete",
"edit": "Edit",
"add": "Add",
"submit": "Submit",
"back": "Back",
"next": "Next",
"confirm": "Confirm"
},
"units": {
"metric": "Metric",
"imperial": "Imperial",
"ml": "ml",
"oz": "oz",
"cm": "cm",
"in": "in",
"kg": "kg",
"lb": "lb"
}
}
```
---
## Phase 3: Custom Hooks
### 3.1 useTranslation Hook
**File**: `hooks/useTranslation.ts` (NEW)
- Re-export from react-i18next with type safety
- Custom hook for easier usage
### 3.2 useLocale Hook
**File**: `hooks/useLocale.ts` (NEW)
- Get current locale
- Change locale
- Get available locales
- Get locale display name
### 3.3 useFormatting Hook
**File**: `hooks/useFormatting.ts` (NEW)
- Format dates based on locale
- Format numbers based on locale
- Format currency based on locale
- Format units based on preference
---
## Phase 4: Measurement Unit Preference
### 4.1 Backend Schema Update
**File**: `src/database/migrations/V0XX_add_measurement_preference.sql` (NEW)
```sql
ALTER TABLE users ADD COLUMN measurement_unit VARCHAR(10) DEFAULT 'metric';
-- Values: 'metric' or 'imperial'
```
### 4.2 Update User Entity
**File**: `src/database/entities/user.entity.ts` (MODIFY)
Add field:
```typescript
@Column({ name: 'measurement_unit', default: 'metric' })
measurementUnit: 'metric' | 'imperial';
```
### 4.3 Update User DTOs
**Files**:
- `src/modules/auth/dto/register.dto.ts` (MODIFY)
- `src/modules/auth/dto/update-profile.dto.ts` (MODIFY)
Add optional `measurementUnit` field.
### 4.4 API Endpoints
**File**: `src/modules/auth/auth.controller.ts` (MODIFY)
- PATCH `/api/v1/auth/profile` - Include measurementUnit in update
---
## Phase 5: Frontend User Preferences
### 5.1 Redux State
**File**: `store/slices/userSlice.ts` (MODIFY)
Add to user state:
```typescript
interface UserState {
// ... existing fields
language: string;
measurementUnit: 'metric' | 'imperial';
}
```
### 5.2 Language Selector Component
**File**: `components/settings/LanguageSelector.tsx` (NEW)
Features:
- Dropdown with 5 language options
- Flag icons for each language
- Updates user preference on change
- Persists to backend
- Updates i18n instance
### 5.3 Measurement Unit Selector Component
**File**: `components/settings/MeasurementUnitSelector.tsx` (NEW)
Features:
- Toggle or dropdown (Metric/Imperial)
- Updates user preference on change
- Persists to backend
- Shows example conversions
---
## Phase 6: Onboarding Flow Integration
### 6.1 Update Onboarding Page
**File**: `app/(auth)/onboarding/page.tsx` (MODIFY)
Add new steps:
1. Language selection (Step 2)
2. Measurement unit selection (Step 3)
### 6.2 Onboarding Language Step Component
**File**: `components/onboarding/LanguageStep.tsx` (NEW)
- Large, visual language selector
- Show language names in native script
- Save to Redux state
- Update i18n immediately
### 6.3 Onboarding Measurement Step Component
**File**: `components/onboarding/MeasurementStep.tsx` (NEW)
- Visual metric/imperial selector
- Show examples (ml vs oz, cm vs in)
- Save to Redux state
### 6.4 Update Onboarding API Call
**File**: `app/(auth)/onboarding/page.tsx` (MODIFY)
Include language and measurementUnit in profile update call.
---
## Phase 7: Settings Page Integration
### 7.1 Update Settings Page
**File**: `app/settings/page.tsx` (MODIFY)
Add new sections:
- **Language & Region** section
- Language selector
- Measurement unit selector
### 7.2 Settings UI Components
**Files**:
- `components/settings/LanguageSettings.tsx` (NEW)
- `components/settings/MeasurementSettings.tsx` (NEW)
---
## Phase 8: Unit Conversion Utilities
### 8.1 Conversion Utility
**File**: `lib/units/conversions.ts` (NEW)
Functions:
- `mlToOz(ml: number): number`
- `ozToMl(oz: number): number`
- `kgToLb(kg: number): number`
- `lbToKg(lb: number): number`
- `cmToIn(cm: number): number`
- `inToCm(in: number): number`
### 8.2 Format Unit Hook
**File**: `hooks/useUnitFormat.ts` (NEW)
- Get user's measurement preference
- Format value with correct unit
- Convert between units
- Display with locale-specific formatting
---
## Phase 9: Apply Localization Throughout App
### 9.1 Update All Pages
Replace hardcoded strings with translation keys:
**Priority Pages**:
1. **Dashboard** (`app/page.tsx`)
- Nav items, headings, button labels
- Activity type labels
- Summary labels
2. **Authentication** (`app/(auth)/`)
- Login, Register, Forgot Password
- Form labels, placeholders, errors
3. **Tracking** (`app/track/`)
- Feeding, Sleep, Diaper pages
- Form labels, unit labels
- Apply unit conversions
4. **Children** (`app/children/page.tsx`)
- Child management labels
- Form fields
5. **Family** (`app/family/page.tsx`)
- Family management labels
6. **AI Assistant** (`app/ai-assistant/page.tsx`)
- Chat interface labels
- Placeholder text
7. **Analytics** (`app/analytics/page.tsx`)
- Chart labels, insights
8. **Settings** (`app/settings/page.tsx`)
- All settings labels
### 9.2 Update Components
Replace hardcoded strings in shared components:
- `components/layouts/AppShell/` (navigation)
- `components/common/` (buttons, dialogs)
- `components/features/` (activity cards, etc.)
---
## Phase 10: Date/Time Localization
### 10.1 Date Formatting Utility
**File**: `lib/i18n/date-formats.ts` (NEW)
Using date-fns with locales:
```typescript
import { format } from 'date-fns';
import { enUS, es, fr, ptBR, zhCN } from 'date-fns/locale';
const locales = { en: enUS, es, fr, pt: ptBR, zh: zhCN };
export function formatDate(date: Date, formatStr: string, locale: string) {
return format(date, formatStr, { locale: locales[locale] });
}
```
### 10.2 Update Date Displays
Apply locale-aware date formatting:
- Activity timestamps
- Child birth dates
- Analytics date ranges
---
## Phase 11: Number Localization
### 11.1 Number Formatting Utility
**File**: `lib/i18n/number-formats.ts` (NEW)
Using Intl.NumberFormat:
```typescript
export function formatNumber(value: number, locale: string) {
return new Intl.NumberFormat(locale).format(value);
}
export function formatDecimal(value: number, locale: string, decimals = 1) {
return new Intl.NumberFormat(locale, {
minimumFractionDigits: decimals,
maximumFractionDigits: decimals,
}).format(value);
}
```
### 11.2 Apply Number Formatting
- Activity amounts (ml/oz)
- Sleep duration (hours)
- Weight/height measurements
---
## Phase 12: Tracking Forms with Unit Preferences
### 12.1 Update Feeding Form
**File**: `app/track/feeding/page.tsx` (MODIFY)
- Show ml or oz input based on preference
- Convert to metric for API storage
- Display in user's preferred unit
### 12.2 Update Child Profile Form
**File**: `app/children/page.tsx` (MODIFY)
- Weight input (kg/lb)
- Height input (cm/in)
- Convert for API storage
### 12.3 Universal Input Component
**File**: `components/forms/UnitInput.tsx` (NEW)
Props:
- `type: 'volume' | 'weight' | 'height'`
- `value: number`
- `onChange: (value: number) => void`
- Auto-convert based on user preference
---
## Phase 13: Translation Files Content
### 13.1 English Translations (Base)
**Estimate**: ~500-800 translation keys across all files
Create complete English translations for:
- All UI strings
- All form labels/placeholders
- All error messages
- All validation messages
### 13.2 Professional Translation
**Recommendation**: Use professional translation service for:
- Spanish (es-ES)
- French (fr-FR)
- Portuguese (pt-BR)
- Simplified Chinese (zh-CN)
**Alternative**: Use AI-assisted translation + native speaker review
---
## Phase 14: Testing
### 14.1 Translation Coverage Test
- Verify all strings are externalized
- No hardcoded English strings remain
- All translation keys exist in all languages
### 14.2 Language Switching Test
- Switch between all 5 languages
- Verify UI updates correctly
- Verify no layout breaks
### 14.3 Unit Conversion Test
- Test all conversions (ml↔oz, kg↔lb, cm↔in)
- Verify correct rounding
- Verify storage in consistent units (metric)
### 14.4 Date/Time Test
- Verify dates display correctly per locale
- Test all date formats (short, long, relative)
### 14.5 RTL Support (Future)
**Note**: Chinese doesn't require RTL, but document for future Arabic support
---
## Phase 15: Documentation
### 15.1 Update Implementation Gaps
**File**: `docs/implementation-gaps.md` (MODIFY)
Mark localization as ✅ COMPLETED
### 15.2 Developer Guide
**File**: `docs/LOCALIZATION_GUIDE.md` (NEW)
Document:
- How to add new translation keys
- How to add new languages
- Translation file structure
- Best practices
### 15.3 Update User Documentation
Document language and measurement preferences in user guide
---
## Implementation Order
### Day 1: Framework & Structure
1. ✅ Install dependencies
2. ✅ Create i18n configuration
3. ✅ Create i18n provider
4. ✅ Create hooks (useTranslation, useLocale, useFormatting)
5. ✅ Set up translation file structure
6. ✅ Create English base translations (common, auth, dashboard)
### Day 2: Preferences & Components
7. ✅ Backend schema update for measurementUnit
8. ✅ Update User entity and DTOs
9. ✅ Create LanguageSelector component
10. ✅ Create MeasurementUnitSelector component
11. ✅ Update onboarding flow
12. ✅ Update settings page
13. ✅ Create unit conversion utilities
### Day 3: Apply Throughout App
14. ✅ Update all pages with translations
15. ✅ Apply unit conversions to tracking forms
16. ✅ Apply date/time localization
17. ✅ Complete all English translations
18. ✅ Test language switching
19. ✅ Test unit conversions
### Post-Implementation (Optional)
20. Professional translation for other 4 languages
21. Native speaker review
22. Documentation updates
---
## Key Files Checklist
### New Files (27 total)
- [ ] `lib/i18n/config.ts`
- [ ] `components/providers/I18nProvider.tsx`
- [ ] `hooks/useLocale.ts`
- [ ] `hooks/useFormatting.ts`
- [ ] `hooks/useUnitFormat.ts`
- [ ] `lib/units/conversions.ts`
- [ ] `lib/i18n/date-formats.ts`
- [ ] `lib/i18n/number-formats.ts`
- [ ] `components/settings/LanguageSelector.tsx`
- [ ] `components/settings/MeasurementUnitSelector.tsx`
- [ ] `components/settings/LanguageSettings.tsx`
- [ ] `components/settings/MeasurementSettings.tsx`
- [ ] `components/onboarding/LanguageStep.tsx`
- [ ] `components/onboarding/MeasurementStep.tsx`
- [ ] `components/forms/UnitInput.tsx`
- [ ] `locales/en/*.json` (11 files)
- [ ] `locales/es/*.json` (11 files - future)
- [ ] `locales/fr/*.json` (11 files - future)
- [ ] `locales/pt/*.json` (11 files - future)
- [ ] `locales/zh/*.json` (11 files - future)
- [ ] `src/database/migrations/V0XX_add_measurement_preference.sql`
- [ ] `docs/LOCALIZATION_GUIDE.md`
### Modified Files (15 total)
- [ ] `package.json`
- [ ] `app/layout.tsx`
- [ ] `store/slices/userSlice.ts`
- [ ] `src/database/entities/user.entity.ts`
- [ ] `src/modules/auth/dto/register.dto.ts`
- [ ] `src/modules/auth/dto/update-profile.dto.ts`
- [ ] `app/(auth)/onboarding/page.tsx`
- [ ] `app/settings/page.tsx`
- [ ] `app/page.tsx` (dashboard)
- [ ] `app/track/feeding/page.tsx`
- [ ] `app/track/sleep/page.tsx`
- [ ] `app/track/diaper/page.tsx`
- [ ] `app/children/page.tsx`
- [ ] `app/family/page.tsx`
- [ ] `docs/implementation-gaps.md`
---
## Success Criteria
### ✅ Functionality
- [ ] All 5 languages selectable and functional
- [ ] Language preference persists across sessions
- [ ] Measurement unit preference persists across sessions
- [ ] All UI strings externalized (no hardcoded English)
- [ ] Unit conversions work correctly (ml↔oz, kg↔lb, cm↔in)
- [ ] Dates display correctly per locale
- [ ] Numbers format correctly per locale
### ✅ User Experience
- [ ] Language can be selected in onboarding
- [ ] Language can be changed in settings
- [ ] Measurement unit can be selected in onboarding
- [ ] Measurement unit can be changed in settings
- [ ] UI updates immediately when language changes
- [ ] No layout breaks when changing languages
- [ ] Form inputs show correct units based on preference
### ✅ Technical
- [ ] All translation keys defined
- [ ] No missing translation warnings
- [ ] Type-safe translation usage (TypeScript)
- [ ] Backend stores preferences correctly
- [ ] Redux state syncs with backend
---
## Notes
### Translation Best Practices
1. Use namespaces to organize translations
2. Use interpolation for dynamic values: `t('welcome', { name })`
3. Use pluralization: `t('items', { count })`
4. Keep keys descriptive: `auth.login.emailLabel` not `auth.e1`
5. Avoid concatenation, use complete sentences
### Unit Conversion Strategy
- **Storage**: Always store in metric (ml, kg, cm) in database
- **Display**: Convert to user's preferred unit for display
- **Input**: Accept user's preferred unit, convert to metric before API call
- **Consistency**: Ensure all measurements use the same preference
### Performance Considerations
- Lazy load translation files per namespace
- Cache translations in browser
- Preload critical translations (common, errors)
### Future Enhancements
- Add more languages (Arabic, German, Hindi, etc.)
- Add more units (temperature: °C/°F)
- Add regional date formats (MM/DD vs DD/MM)
- Add time format preferences (12h vs 24h)
---
**Total Estimated Effort**: 2-3 days
**Complexity**: Medium
**Priority**: HIGH (Pre-Launch)
**Dependencies**: None (can start immediately)