# 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)