feat(language): add progressive locale code truncation to normalizeLanguage

src/middleware/language/index.test.ts

@@ -77,6 +77,82 @@ describe('languageDetector', () => {
       expect(await res.text()).toBe('fr')
     })
 
+    it('should fallback to language code when locale code is not in supportedLanguages', async () => {
+      const app = createTestApp({
+        supportedLanguages: ['en', 'ja'],
+        fallbackLanguage: 'en',
+        order: ['header'],
+      })
+
+      const res = await app.request('/', {
+        headers: {
+          'accept-language': 'ja-JP',
+        },
+      })
+      expect(await res.text()).toBe('ja')
+    })
+
+    it('should match after multiple truncations', async () => {
+      const app = createTestApp({
+        supportedLanguages: ['zh-Hant', 'en'],
+        fallbackLanguage: 'en',
+        order: ['header'],
+      })
+
+      const res = await app.request('/', {
+        headers: {
+          'accept-language': 'zh-Hant-CN',
+        },
+      })
+      expect(await res.text()).toBe('zh-Hant')
+    })
+
+    it('should fallback when truncation does not match any supported language', async () => {
+      const app = createTestApp({
+        supportedLanguages: ['en', 'ja'],
+        fallbackLanguage: 'en',
+        order: ['header'],
+      })
+
+      const res = await app.request('/', {
+        headers: {
+          'accept-language': 'ko-KR',
+        },
+      })
+      expect(await res.text()).toBe('en')
+    })
+
+    it('should prefer exact match over truncated match', async () => {
+      const app = createTestApp({
+        supportedLanguages: ['fr', 'fr-CA'],
+        fallbackLanguage: 'fr',
+        order: ['header'],
+      })
+
+      const res = await app.request('/', {
+        headers: {
+          'accept-language': 'fr-CA',
+        },
+      })
+      expect(await res.text()).toBe('fr-CA')
+    })
+
+    it('should handle case-insensitive truncation matching', async () => {
+      const app = createTestApp({
+        supportedLanguages: ['en', 'ja'],
+        fallbackLanguage: 'en',
+        order: ['header'],
+        ignoreCase: true,
+      })
+
+      const res = await app.request('/', {
+        headers: {
+          'accept-language': 'JA-JP',
+        },
+      })
+      expect(await res.text()).toBe('ja')
+    })
+
     it('should handle malformed Accept-Language headers', async () => {
       const app = createTestApp({
         supportedLanguages: ['en', 'fr'],

src/middleware/language/language.ts

@@ -100,8 +100,23 @@ export const normalizeLanguage = (
       options.ignoreCase ? l.toLowerCase() : l
     )
 
-    const matchedLang = compSupported.find((l) => l === compLang)
-    return matchedLang ? options.supportedLanguages[compSupported.indexOf(matchedLang)] : undefined
+    // Exact match
+    const exactIndex = compSupported.indexOf(compLang)
+    if (exactIndex !== -1) {
+      return options.supportedLanguages[exactIndex]
+    }
+
+    // Progressive truncation (RFC 4647 Lookup)
+    const parts = compLang.split('-')
+    for (let i = parts.length - 1; i > 0; i--) {
+      const candidate = parts.slice(0, i).join('-')
+      const prefixIndex = compSupported.indexOf(candidate)
+      if (prefixIndex !== -1) {
+        return options.supportedLanguages[prefixIndex]
+      }
+    }
+
+    return undefined
   } catch {
     return undefined
   }