fix: use region-specific translations for default locale instead of base language override

When a monolingual site uses a region-specific locale as the default (e.g., `zh-TW`),
`buildResources` for the default locale was loading both `builtinTranslations['zh-TW']`
and `builtinTranslations['zh']` as separate layers. Since `buildResources` applies later
layers over earlier ones, the base language (`zh` → Simplified Chinese) was overwriting
the region-specific translations (`zh-TW` → Traditional Chinese).

This fix aligns the default locale behavior with the locales loop (line 37), using `||`
so `stripLangRegion` result is only used as a fallback when the specific locale has no
built-in translations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MilesChou

packages/starlight/__tests__/i18n-single-root-locale/translations-fs.test.ts

@@ -28,6 +28,20 @@ describe('createTranslationSystemFromFs', () => {
 		expect(t('page.editLink')).toMatchInlineSnapshot('"Changer cette page"');
 	});
 
+	test('uses region-specific translations instead of base language fallback for default locale', async () => {
+		const useTranslations = await createTranslationSystemFromFs(
+			{
+				locales: undefined,
+				defaultLocale: { label: '繁體中文', lang: 'zh-TW', dir: 'ltr' },
+			},
+			// Using non-existent `_src/` to ignore custom files in this test fixture.
+			{ srcDir: new URL('./_src/', import.meta.url) }
+		);
+		const t = useTranslations('zh-TW');
+		// Should use zh-TW (Traditional Chinese) "搜尋", not zh (Simplified Chinese) "搜索"
+		expect(t('search.label')).toBe('搜尋');
+	});
+
 	test('returns translation for unknown language', async () => {
 		const useTranslations = await createTranslationSystemFromFs(
 			{

packages/starlight/utils/createTranslationSystem.ts

@@ -22,8 +22,7 @@ export async function createTranslationSystem<T extends i18nSchemaOutput>(
 
 	const translations = {
 		[defaultLocale]: buildResources(
-			builtinTranslations[defaultLocale],
-			builtinTranslations[stripLangRegion(defaultLocale)],
+			builtinTranslations[defaultLocale] || builtinTranslations[stripLangRegion(defaultLocale)],
 			pluginTranslations[defaultLocale],
 			userTranslations[defaultLocale]
 		),