Fix regional locale detection

src/i18n/detectLocale.test.ts

@@ -0,0 +1,74 @@
+import { assertEquals } from "@test/assert";
+import { detectLocale } from "./detectLocale";
+
+function runTests() {
+    assertEquals(
+        detectLocale("zh-TW,zh;q=0.9,en-US;q=0.8,en;q=0.7,ja;q=0.6"),
+        "zh-TW",
+        "An exact regional match should take precedence over a language fallback"
+    );
+    assertEquals(
+        detectLocale("ZH-tw"),
+        "zh-TW",
+        "Locale matching should be case-insensitive"
+    );
+    assertEquals(
+        detectLocale("  zh-TW ; q=1 , zh-CN;q=0.8 "),
+        "zh-TW",
+        "Whitespace and quality parameters should not prevent an exact match"
+    );
+    assertEquals(
+        detectLocale("zh-CN,zh-TW;q=0.9"),
+        "zh-CN",
+        "Simplified Chinese should still match exactly"
+    );
+    assertEquals(
+        detectLocale("zh"),
+        "zh-CN",
+        "A generic Chinese preference should retain the existing fallback"
+    );
+    assertEquals(
+        detectLocale("en-GB,en;q=0.9"),
+        "en-US",
+        "An unsupported region should fall back to a supported locale for the language"
+    );
+    assertEquals(
+        detectLocale("ja-JP,zh-TW;q=0.9"),
+        "zh-TW",
+        "The next preference should be used when a language is unsupported"
+    );
+    assertEquals(
+        detectLocale("zh-CN;q=0.5,zh-TW;q=0.9"),
+        "zh-TW",
+        "Preferences should be evaluated by quality"
+    );
+    assertEquals(
+        detectLocale("zh-TW;q=0,fr-FR;q=0.8"),
+        "fr-FR",
+        "Locales with zero quality should be excluded"
+    );
+    assertEquals(
+        detectLocale("*,de-DE;q=0.8"),
+        "de-DE",
+        "A wildcard should not obscure a supported preference"
+    );
+    assertEquals(
+        detectLocale("ja-JP"),
+        undefined,
+        "An unsupported language should not match"
+    );
+    assertEquals(
+        detectLocale(""),
+        undefined,
+        "An empty Accept-Language header should not match"
+    );
+
+    console.log("All locale detection tests passed!");
+}
+
+try {
+    runTests();
+} catch (error) {
+    console.error("Locale detection test failed:", error);
+    process.exit(1);
+}

src/i18n/detectLocale.ts

@@ -0,0 +1,48 @@
+import { Locale, locales } from "./config";
+
+export function detectLocale(acceptLanguage: string): Locale | undefined {
+    const browserLocales = acceptLanguage
+        .split(",")
+        .map((entry, index) => {
+            const [locale, ...parameters] = entry.trim().split(";");
+            const qualityParameter = parameters.find((parameter) =>
+                parameter.trim().toLowerCase().startsWith("q=")
+            );
+            const quality = qualityParameter
+                ? Number(qualityParameter.trim().slice(2))
+                : 1;
+
+            return {
+                locale: locale.trim().toLowerCase(),
+                quality,
+                index
+            };
+        })
+        .filter(
+            ({ locale, quality }) =>
+                locale && locale !== "*" && quality > 0 && quality <= 1
+        )
+        .sort(
+            (left, right) =>
+                right.quality - left.quality || left.index - right.index
+        );
+
+    for (const { locale: browserLocale } of browserLocales) {
+        const exactMatch = locales.find(
+            (locale) => locale.toLowerCase() === browserLocale
+        );
+        if (exactMatch) {
+            return exactMatch;
+        }
+
+        const browserLanguage = browserLocale.split("-")[0];
+        const languageMatch = locales.find(
+            (locale) => locale.split("-")[0].toLowerCase() === browserLanguage
+        );
+        if (languageMatch) {
+            return languageMatch;
+        }
+    }
+
+    return undefined;
+}

src/services/locale.ts

@@ -2,6 +2,7 @@
 
 import { cookies, headers } from "next/headers";
 import { Locale, defaultLocale, locales } from "@/i18n/config";
+import { detectLocale } from "@/i18n/detectLocale";
 import { internal } from "@app/lib/api";
 import { authCookieHeader } from "@app/lib/api/cookies";
 
@@ -47,12 +48,7 @@ export async function getUserLocale(): Promise<Locale> {
     const acceptLang = headerList.get("accept-language");
 
     if (acceptLang) {
-        const browserLang = acceptLang.split(",")[0];
-        const matched = locales.find((locale) =>
-            browserLang
-                .toLowerCase()
-                .startsWith(locale.split("-")[0].toLowerCase())
-        );
+        const matched = detectLocale(acceptLang);
         if (matched) {
             return matched;
         }