fix: reduce excessive MISSING_TRANSLATION warnings for fallback locales

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

@@ -1,6 +1,15 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { getLocale } 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>) {
   (window as unknown as Record<string, unknown>).appConfig = {
@@ -61,17 +70,40 @@ describe('getLocale', () => {
 });
 
 describe('loadMessages', () => {
-  it('returns empty object for English locale', async () => {
+  afterEach(() => {
+    vi.doUnmock('./compiled/en.json');
+    vi.doUnmock('./compiled/zh-CN.json');
+    vi.resetModules();
+  });
+
+  async function loadMessagesWithMockedCatalogs() {
+    vi.resetModules();
+    vi.doMock('./compiled/en.json', () => ({ default: englishMessages }));
+    vi.doMock('./compiled/zh-CN.json', () => ({ default: zhCnMessages }));
+
     const { loadMessages } = await import('./index');
+    return loadMessages;
+  }
+
+  it('returns compiled English messages for English locale', async () => {
+    const loadMessages = await loadMessagesWithMockedCatalogs();
     const messages = await loadMessages('en');
-    expect(messages).toEqual({});
+    expect(messages).toEqual(englishMessages);
   });
 
-  it('returns empty object for unsupported locale (with warning)', async () => {
+  it('merges locale messages over English fallback messages', async () => {
+    const loadMessages = await loadMessagesWithMockedCatalogs();
+    const messages = await loadMessages('zh-CN');
+
+    expect(messages.shared).toBe(zhCnMessages.shared);
+    expect(messages.englishOnly).toBe(englishMessages.englishOnly);
+  });
+
+  it('returns English messages for unsupported locale (with warning)', async () => {
     const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
-    const { loadMessages } = await import('./index');
+    const loadMessages = await loadMessagesWithMockedCatalogs();
     const messages = await loadMessages('xx');
-    expect(messages).toEqual({});
+    expect(messages).toEqual(englishMessages);
     expect(warnSpy).toHaveBeenCalledWith(expect.stringContaining('No message catalog found'));
     warnSpy.mockRestore();
   });

ui/desktop/src/i18n/index.ts

@@ -90,24 +90,35 @@
 /** Base language for loading message catalogs (e.g. "en"). */
 export const currentMessageLocale = resolvedLocale.messageLocale;
 
+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>> {
+  const englishMessages = await loadCompiledMessages('en');
+
   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/${locale}.json`);
-    return mod.default ?? mod;
+    const messages = await loadCompiledMessages(locale);
+    return {
+      ...englishMessages,
+      ...messages,
+    };
   } catch {
     console.warn(
       `[i18n] No message catalog found for locale "${locale}", falling back to English.`
     );
-    return {};
+    return englishMessages;
   }
 }