diff --git a/src/__tests__/i18n.test.ts b/src/__tests__/i18n.test.ts
new file mode 100644
index 00000000..84d05157
--- /dev/null
+++ b/src/__tests__/i18n.test.ts
@@ -0,0 +1,91 @@
+import { describe, expect, test } from 'bun:test';
+
+describe('I18n', () => {
+  // Use a helper function to import the real implementation
+  // since `../i18n` is globally mocked in `setup.ts`
+  const getRealI18n = async () => {
+    // Generate a random cache-buster so we get a fresh instance if needed,
+    // although singleton is usually fine, the `setup.ts` mock needs to be bypassed.
+    const cacheKey = 'real';
+    const module = await import(`../i18n?${cacheKey}`);
+    return { I18n: module.I18n, i18nSingleton: module.default };
+  };
+
+  test('setLocale and getLocale', async () => {
+    const { I18n } = await getRealI18n();
+    const i18n = new I18n();
+
+    expect(i18n.getLocale()).toBe('en'); // Default is 'en'
+
+    i18n.setLocale('zh');
+    expect(i18n.getLocale()).toBe('zh');
+
+    i18n.setLocale('en');
+    expect(i18n.getLocale()).toBe('en');
+  });
+
+  test('basic translations and fallback', async () => {
+    const { I18n } = await getRealI18n();
+    const i18n = new I18n();
+
+    // English translation
+    i18n.setLocale('en');
+    expect(i18n.t('checking_update')).toBe('Checking for updates...');
+
+    // Chinese translation
+    i18n.setLocale('zh');
+    expect(i18n.t('checking_update')).toBe('正在检查更新...');
+
+    // Fallback logic
+    // Add translation only to 'zh', check if 'en' falls back to 'zh'
+    i18n.addTranslations('zh', { 'only_in_zh': '中文专属' });
+    i18n.setLocale('en');
+    expect(i18n.t('only_in_zh' as any)).toBe('中文专属');
+
+    // Add translation only to 'en', check if 'zh' falls back to 'en'
+    i18n.addTranslations('en', { 'only_in_en': 'English Only' });
+    i18n.setLocale('zh');
+    expect(i18n.t('only_in_en' as any)).toBe('English Only');
+
+    // Key missing in both locales
+    expect(i18n.t('missing_key' as any)).toBe('missing_key');
+  });
+
+  test('string interpolation', async () => {
+    const { I18n } = await getRealI18n();
+    const i18n = new I18n();
+
+    i18n.setLocale('en');
+
+    // Normal interpolation
+    expect(i18n.t('download_progress', { progress: 50 })).toBe('Download progress: 50%');
+
+    // Multiple interpolations
+    expect(i18n.t('retry_count', { count: 1, max: 3 })).toBe('Retry attempt: 1/3');
+
+    // Interpolation with missing values (should leave placeholder as is)
+    expect(i18n.t('download_progress')).toBe('Download progress: {{progress}}%');
+
+    // Invalid interpolation syntax (spaces) are ignored
+    i18n.addTranslations('en', { 'invalid_syntax': 'Test {{ key }} here' });
+    expect(i18n.t('invalid_syntax' as any, { key: 'value' })).toBe('Test {{ key }} here');
+  });
+
+  test('addTranslations', async () => {
+    const { I18n } = await getRealI18n();
+    const i18n = new I18n();
+
+    i18n.addTranslations('en', { 'new_key': 'New Value' });
+    i18n.setLocale('en');
+
+    expect(i18n.t('new_key' as any)).toBe('New Value');
+  });
+
+  test('singleton export', async () => {
+    const { i18nSingleton } = await getRealI18n();
+
+    // Verify it works
+    expect(i18nSingleton.getLocale()).toBeDefined();
+    expect(i18nSingleton.t('checking_update')).toBeDefined();
+  });
+});