Fix runtime default locale resolution

tests/locale.test.ts

@@ -1,8 +1,22 @@
 import { describe, expect, it } from "vitest";
 
-import { getLocalizedPathWithDefault } from "../lib/locale";
+import { FALLBACK_LOCALE, getLocalizedPathWithDefault, isSupportedLocale, resolveLocale } from "../lib/locale";
 
 describe("locale path helpers", () => {
+  it("recognizes supported locales explicitly", () => {
+    expect(isSupportedLocale("ar")).toBe(true);
+    expect(isSupportedLocale("en")).toBe(true);
+    expect(isSupportedLocale("de")).toBe(true);
+    expect(isSupportedLocale("fr")).toBe(false);
+    expect(isSupportedLocale(undefined)).toBe(false);
+  });
+
+  it("resolves invalid locales with an explicit fallback", () => {
+    expect(resolveLocale("ar", "de")).toBe("ar");
+    expect(resolveLocale("fr", "en")).toBe("en");
+    expect(resolveLocale("", FALLBACK_LOCALE)).toBe("de");
+  });
+
   it("keeps the configured default locale on the bare domain", () => {
     expect(getLocalizedPathWithDefault("ar", "/", "ar")).toBe("/");
     expect(getLocalizedPathWithDefault("de", "/", "ar")).toBe("/de");