fix: reduce excessive MISSING_TRANSLATION warnings for fallback locales

ui/desktop/src/i18n/i18n.test.ts

@@ -1,5 +1,14 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
-import { getLocale } from './index';
+import { getLocale, loadMessagesWithCatalogLoader } from './index';
+
+const englishMessages = {
+  shared: 'English shared message',
+  englishOnly: 'English fallback message',
+};
+
+const zhCnMessages = {
+  shared: 'Chinese shared message',
+};
 
 // Helper to mock window.appConfig for tests
 function mockAppConfig(values: Record<string, unknown>) {
@@ -61,18 +70,53 @@ describe('getLocale', () => {
 });
 
 describe('loadMessages', () => {
-  it('returns empty object for English locale', async () => {
-    const { loadMessages } = await import('./index');
-    const messages = await loadMessages('en');
-    expect(messages).toEqual({});
+  it('returns compiled English messages for English locale', async () => {
+    const loadCatalog = vi.fn().mockResolvedValue(englishMessages);
+    const messages = await loadMessagesWithCatalogLoader('en', loadCatalog);
+
+    expect(messages).toEqual(englishMessages);
+    expect(loadCatalog).toHaveBeenCalledOnce();
+    expect(loadCatalog).toHaveBeenCalledWith('en');
   });
 
-  it('returns empty object for unsupported locale (with warning)', async () => {
+  it('merges locale messages over English fallback messages', async () => {
+    const loadCatalog = vi.fn(async (locale: string) =>
+      locale === 'zh-CN' ? zhCnMessages : englishMessages
+    );
+    const messages = await loadMessagesWithCatalogLoader('zh-CN', loadCatalog);
+
+    expect(messages.shared).toBe(zhCnMessages.shared);
+    expect(messages.englishOnly).toBe(englishMessages.englishOnly);
+    expect(loadCatalog).toHaveBeenCalledWith('en');
+    expect(loadCatalog).toHaveBeenCalledWith('zh-CN');
+  });
+
+  it('returns English messages for unsupported locale (with warning)', async () => {
     const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
-    const { loadMessages } = await import('./index');
-    const messages = await loadMessages('xx');
-    expect(messages).toEqual({});
+    const loadCatalog = vi.fn(async (locale: string) => {
+      if (locale === 'xx') {
+        throw new Error('missing catalog');
+      }
+      return englishMessages;
+    });
+    const messages = await loadMessagesWithCatalogLoader('xx', loadCatalog);
+
+    expect(messages).toEqual(englishMessages);
     expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining('No message catalog found'));
     warnSpy.mockRestore();
   });
+
+  it('falls back to default messages when the English catalog is unavailable', async () => {
+    const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const loadCatalog = vi.fn(async () => {
+      throw new Error('missing catalog');
+    });
+    const messages = await loadMessagesWithCatalogLoader('en', loadCatalog);
+
+    expect(messages).toEqual({});
+    expect(warnSpy).toHaveBeenCalledWith(
+      '[i18n] No English fallback catalog found; missing messages will use source defaultMessage values.'
+    );
+    warnSpy.mockRestore();
+  });
 });

ui/desktop/src/i18n/index.ts

@@ -90,24 +90,51 @@ export const currentLocale = resolvedLocale.locale;
 /** Base language for loading message catalogs (e.g. "en"). */
 export const currentMessageLocale = resolvedLocale.messageLocale;
 
+export async function loadCompiledMessages(locale: string): Promise<Record<string, string>> {
+  const mod = await import(`./compiled/${locale}.json`);
+  return (mod.default ?? mod) as Record<string, string>;
+}
+
 /**
  * Load compiled messages for a given locale.
- * Returns an empty object for English (react-intl uses defaultMessage as fallback).
+ * English messages are always loaded as the fallback catalog so regional
+ * English locales can keep their locale for date/number formatting without
+ * triggering missing translation warnings for every message.
  */
 export async function loadMessages(locale: string): Promise<Record<string, string>> {
+  return loadMessagesWithCatalogLoader(locale, loadCompiledMessages);
+}
+
+export async function loadMessagesWithCatalogLoader(
+  locale: string,
+  loadCatalog: (locale: string) => Promise<Record<string, string>>
+): Promise<Record<string, string>> {
+  let englishMessages: Record<string, string>;
+
+  try {
+    englishMessages = await loadCatalog('en');
+  } catch {
+    console.warn(
+      '[i18n] No English fallback catalog found; missing messages will use source defaultMessage values.'
+    );
+    englishMessages = {};
+  }
+
   if (locale === 'en') {
-    // English strings live in source code as defaultMessage — no catalog needed.
-    return {};
+    return englishMessages;
   }
 
   try {
     // Dynamic import so compiled translation bundles are code-split.
-    const mod = await import(`./compiled/${locale}.json`);
-    return mod.default ?? mod;
+    const messages = await loadCatalog(locale);
+    return {
+      ...englishMessages,
+      ...messages,
+    };
   } catch {
     console.warn(
-      `[i18n] No message catalog found for locale "${locale}", falling back to English.`
+      `[i18n] No message catalog found for locale "${locale}"; using fallback messages.`
     );
-    return {};
+    return englishMessages;
   }
 }