# Enterprise i18n Architecture (`@repo/core-i18n`)

[← Back to Root](../../README.md)

A highly decoupled, type-safe internationalization engine for the Eigen Monorepo. 

It uses a **Hybrid Namespace Strategy**:
1. **Centralized Engine**: Setup, local persistence (`@repo/core-storage`), and global words (`common`).
2. **Decentralized Dictionaries**: Feature-specific translations (`booking`, `billing`) live inside the application modules and are lazy-loaded.

This architecture strictly adheres to **Inversion of Control (IoC)**. The core engine handles local state and performance, but leaves API and networking decisions entirely to the consuming applications.

---

## Overview Architecture

```mermaid
graph TD
    subgraph Apps ["apps/* (App Autonomy)"]
        UI[React Components]
        DICT[Feature Dictionaries<br/>e.g., booking.json]
    end

    subgraph Core ["@repo/core-i18n (Engine)"]
        I18N((i18next Instance))
        STORE[(core-storage)]
        COMMON[Common Vocabulary]
    end

    subgraph Backend ["Backend API"]
        SYNC[Language Sync Endpoint]
        TENANT[Tenant Config Endpoint]
    end

    UI -->|uses useTranslation| I18N
    DICT -.->|lazy loads| I18N
    COMMON -->|preloads| I18N
    I18N <-->|reads/persists| STORE
    
    I18N -->|changeLanguage sync| SYNC
    SYNC -.->|fails? rollback| I18N
    
    TENANT -.->|applyTenantOverrides| I18N
    
    style I18N fill:#4263eb,color:#fff,stroke:#fff
    style Apps fill:#f8f9fa,stroke:#ced4da
    style Core fill:#f8f9fa,stroke:#ced4da
```

---

## 1. App-Level Setup (Bootstrap)

Initialize the engine *before* your React application mounts to prevent UI flashing.

```tsx
// apps/web/src/main.tsx
import { StrictMode } from 'react';
import { createRoot } from 'react-dom/client';
import { setupI18n } from '@repo/core-i18n';
import App from './app';

async function bootstrap() {
  // Synchronously reads preferred language from storage & inits i18next
  await setupI18n();

  createRoot(document.getElementById('app')!).render(
    <StrictMode><App /></StrictMode>,
  );
}
bootstrap();
```

---

## 2. Module-Level Setup (Decentralized Dictionaries)

Dictionaries live right next to the UI components that use them.

### Folder Structure
```text
apps/web/src/apps/modules/booking/
├── presentation/BookingTable.tsx
└── locales/
    ├── id/booking.json
    └── en/booking.json
```

### Lazy Loading & Type Safety
Register the namespace when the component mounts. To get native TypeScript autocomplete for nested keys (e.g., `header.title`), augment the global `react-i18next` types.

**1. Augment Types:**
```ts
// apps/web/src/types/i18next.d.ts
import 'react-i18next';
import type { resources as coreResources } from '@repo/core-i18n/src/setup';
import bookingEn from '../apps/modules/booking/locales/en/booking.json';

declare module 'react-i18next' {
  interface CustomTypeOptions {
    defaultNS: 'common';
    resources: typeof coreResources['en'] & { booking: typeof bookingEn };
  }
}
```

**2. Use in Component:**
```tsx
import { useEffect } from 'react';
import { i18n, useTranslation } from '@repo/core-i18n';
import bookingId from '../locales/id/booking.json';
import bookingEn from '../locales/en/booking.json';

export default function BookingFeature() {
  const { t } = useTranslation(['common', 'booking']);

  useEffect(() => {
    i18n.addResourceBundle('id', 'booking', bookingId, true, false);
    i18n.addResourceBundle('en', 'booking', bookingEn, true, false);
  }, []);

  return <h1>{t('booking:header.title')}</h1>; // Autocomplete works!
}
```

**3. Dynamic Variables (Interpolation):**
```json
// booking.json
{
  "messages": {
    "welcome": "Welcome back, {{name}}! You have {{count}} new bookings."
  }
}
```
```tsx
// Inside component
<h1>{t('booking:messages.welcome', { name: 'Firman', count: 5 })}</h1>
```

---

## 3. Usage Outside React Components (Vanilla TS)

For utility files, API interceptors, or vanilla functions where React hooks cannot be used, import the raw `i18n` instance directly.

```ts
import { i18n } from '@repo/core-i18n';

// Must specify the namespace explicitly if it's not 'common'
export const getErrorMessage = (code: string) => {
  return i18n.t(`booking:errors.${code}`, { defaultValue: 'Unknown Error' });
};
```

---

## 4. Real-World Implementation Flow

The engine supports robust flows for authenticated apps, including Tenant Vocabulary Overrides and Backend Synchronization. 

### A. The Tenant Override Flow (After Login)

If "Company A" calls "Purchasing" -> "Procurement", they shouldn't need a custom build. The backend returns an override config, and the frontend dynamically merges it using `applyTenantOverrides`.

```tsx
// Example inside an AuthProvider or Post-Login useEffect
import { useEffect } from 'react';
import { applyTenantOverrides } from '@repo/core-i18n';
import { api } from '@/api';

export function AuthProvider({ children }) {
  useEffect(() => {
    async function fetchTenantConfig() {
      try {
        // 1. Fetch tenant-specific overrides from the API
        const response = await api.get('/v1/tenant/i18n-config');
        
        // 2. Inject into the engine. 
        // `deep: true` ensures only provided keys are overridden.
        applyTenantOverrides(
          response.data.namespace, 
          response.data.overrides
        );
      } catch (err) {
        console.error("Failed to fetch tenant configuration", err);
      }
    }
    
    fetchTenantConfig();
  }, []);

  return <>{children}</>;
}
```

### B. User Preference Sync (With Rollback)

When a logged-in user changes their language, we update the UI instantly, save it locally, and sync it to the backend. If the backend fails, the engine automatically rolls back.

```tsx
import { changeLanguage } from '@repo/core-i18n';
import { api } from '@/api';

const handleSwitch = async (newLng: string) => {
  try {
    await changeLanguage(newLng, async (lng) => {
      // The core engine waits for this Promise. 
      // If it throws, the UI reverts to the previous language automatically.
      await api.patch('/v1/user/profile', { language: lng });
    });
    toast.success('Language saved!');
  } catch (err) {
    toast.error('Sync failed. Reverted to previous language.');
  }
};
```
> [!NOTE]
> For public pages (like `apps/landing`), simply call `changeLanguage('en')` without the callback function. It will update the UI and local storage instantly without hitting the network.

---

## 5. Backend API Contract (For Backend Engineers)

To support Dynamic Tenant Overrides, the backend must expose an endpoint (e.g., `GET /v1/tenant/i18n-config`).

### Identification
The backend **MUST identify the tenant via the `Authorization` (JWT) header**. The frontend will not send `tenantId` in the query payload to prevent spoofing.

### Expected JSON Response Format
The response must match the structural shape of the frontend dictionary. Because the frontend uses a **Deep Merge** strategy, the backend **only needs to return the specific keys the tenant wishes to override**. 

If the frontend dictionary has `header.title` and `header.subtitle`, and the backend only sends `header.title`, the `subtitle` will safely remain intact.

**Example Request:**
`GET /v1/tenant/i18n-config`
*(Authorization: Bearer eyJhbG...)*

**Expected Response (200 OK):**
```json
{
  "data": {
    "namespace": "booking",
    "overrides": {
      "module_name": "Procurement",
      "header": {
        "title": "Procurement List"
      }
    }
  }
}
```