feat: implement core-i18n package with decentralized namespace support and integrate into landing and web apps

packages/core-i18n/src/index.ts

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+export { setupI18n, type SupportedLanguage } from './setup';
+export { changeLanguage, applyTenantOverrides } from './manager';
+export { useTranslation, Trans } from 'react-i18next';
+export { default as i18n } from 'i18next';

packages/core-i18n/src/locales/en/common.json

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+{
+  "common": {
+    "save": "Save",
+    "cancel": "Cancel",
+    "success": "Success",
+    "error": "Error",
+    "settings": "Settings",
+    "loading": "Loading...",
+    "delete": "Delete",
+    "edit": "Edit"
+  }
+}

packages/core-i18n/src/locales/id/common.json

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+{
+  "common": {
+    "save": "Simpan",
+    "cancel": "Batal",
+    "success": "Sukses",
+    "error": "Galat",
+    "settings": "Pengaturan",
+    "loading": "Memuat...",
+    "delete": "Hapus",
+    "edit": "Ubah"
+  }
+}

packages/core-i18n/src/manager.ts

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+import i18n from 'i18next';
+import { demoSecureStorage, StorageKey } from '@repo/core-storage';
+
+/**
+ * Changes the active language, saves the preference locally, and optionally syncs with the backend.
+ *
+ * @param newLng The new language code (e.g., 'en', 'id')
+ * @param syncCallback An optional callback to sync the preference to the backend. It receives the new language and the previous language.
+ */
+export async function changeLanguage(
+  newLng: string,
+  syncCallback?: (newLng: string, prevLng: string) => Promise<void>
+): Promise<void> {
+  const prevLng = i18n.language;
+  
+  if (prevLng === newLng) return;
+
+  // 1. Update local storage & i18next optimistically
+  await demoSecureStorage.setItem(StorageKey.LOCALE, newLng);
+  await i18n.changeLanguage(newLng);
+
+  // 2. Trigger optional backend sync
+  if (syncCallback) {
+    try {
+      await syncCallback(newLng, prevLng);
+    } catch (error) {
+      console.error('[i18n] Backend sync failed, rolling back language', error);
+      // Rollback on failure
+      await demoSecureStorage.setItem(StorageKey.LOCALE, prevLng);
+      await i18n.changeLanguage(prevLng);
+      throw error; // Rethrow so the caller can show an error toast
+    }
+  }
+}
+
+/**
+ * Injects tenant-specific vocabulary overrides dynamically at runtime.
+ *
+ * Uses a deep-merge strategy. Overrides are applied to the currently active language,
+ * or across all loaded languages if needed.
+ *
+ * @param namespace The i18n namespace to override (e.g., 'common', 'booking')
+ * @param overrides A deeply nested object containing the overridden string keys and values.
+ * @param lng Specific language to override. Defaults to currently active language.
+ */
+export function applyTenantOverrides(
+  namespace: string,
+  overrides: Record<string, unknown>,
+  lng?: string
+): void {
+  const targetLng = lng || i18n.language;
+  
+  // deep: true -> merges with existing keys rather than replacing the whole namespace
+  // overwrite: true -> allows replacing existing specific keys
+  i18n.addResourceBundle(targetLng, namespace, overrides, true, true);
+}

packages/core-i18n/src/react-i18next.d.ts

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+import 'react-i18next';
+import type { resources } from './setup';
+
+declare module 'react-i18next' {
+  interface CustomTypeOptions {
+    defaultNS: 'common';
+    resources: typeof resources['en'];
+  }
+}

packages/core-i18n/src/setup.ts

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+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import { demoSecureStorage, StorageKey } from '@repo/core-storage';
+import commonEn from './locales/en/common.json';
+import commonId from './locales/id/common.json';
+
+const DEFAULT_LANGUAGE = 'id';
+const SUPPORTED_LANGUAGES = ['en', 'id'] as const;
+
+export type SupportedLanguage = (typeof SUPPORTED_LANGUAGES)[number];
+
+export const resources = {
+  en: { common: commonEn.common },
+  id: { common: commonId.common },
+} as const;
+
+/**
+ * Bootstraps the central i18n engine.
+ * 
+ * This reads the preferred locale from secureStorage and initializes
+ * i18next synchronously before React renders.
+ */
+export async function setupI18n(): Promise<void> {
+  let initialLng = DEFAULT_LANGUAGE;
+  try {
+    const storedLng = await demoSecureStorage.getItem<string>(StorageKey.LOCALE);
+    if (storedLng && SUPPORTED_LANGUAGES.includes(storedLng as SupportedLanguage)) {
+      initialLng = storedLng;
+    }
+  } catch (err) {
+    console.warn('[i18n] Failed to read locale from storage', err);
+  }
+
+  await i18n
+    .use(initReactI18next)
+    .init({
+      resources,
+      lng: initialLng,
+      fallbackLng: DEFAULT_LANGUAGE,
+      defaultNS: 'common',
+      interpolation: {
+        escapeValue: false, // React already escapes values
+      },
+    });
+
+  // Apply initial language to the DOM for SEO/Accessibility
+  if (typeof document !== 'undefined') {
+    document.documentElement.lang = i18n.language;
+  }
+
+  // Ensure DOM updates whenever the language changes later
+  i18n.on('languageChanged', (lng) => {
+    if (typeof document !== 'undefined') {
+      document.documentElement.lang = lng;
+    }
+  });
+}