fix: improve support for multi-part languages

src/core/__tests__/i18n.test.ts

@@ -45,6 +45,30 @@ describe("i18n", () => {
 			expect(i18n.language).toBe("en");
 		});
 
+		test.each([
+			{
+				language: "zh-Hant-DE",
+				expected: "zh_TW",
+			},
+			{
+				language: "zh-Hans-DE",
+				expected: "zh_CN",
+			},
+			{
+				language: "en-GB-DE",
+				expected: "en",
+			},
+		])("can parse multi-part language: $language equals $expected", async ({ language, expected }) => {
+			// given.
+			jest.spyOn(window.navigator, "language", "get").mockReturnValue(language);
+
+			// when.
+			const i18n = (await import("../i18n")).default;
+
+			// then.
+			expect(i18n.language).toBe(expected);
+		});
+
 		describe("Simplified/Traditional Chinese", () => {
 			it("should map traditional Chinese zh-TW to zh_TW", async () => {
 				// given.

src/core/i18n.ts

@@ -40,19 +40,16 @@ export class Internationalization {
 	 * {@link https://developer.chrome.com/docs/extensions/reference/i18n/#method-getUILanguage}
 	 */
 	public getUILanguage(): string {
-		const language = window.navigator.language;
+		const [language, region] = window.navigator.language.split("-");
 
-		// Chinese Traditional
-		if (language === "zh-Hant" || language === "zh-TW") {
-			return "zh_TW";
+		// Chinese
+		if (language === "zh") {
+			return region === "Hant" || region === "TW"
+				? "zh_TW" // Traditional (zh-Hant, zh-TW)
+				: "zh_CN"; // Simplified (zh-Hans, zh-CN)
 		}
 
-		// Chinese Simplified
-		if (language === "zh" || language === "zh-Hans" || language === "zh-CN") {
-			return "zh_CN";
-		}
-
-		return window.navigator.language ? window.navigator.language.split("-")[0] : "en";
+		return language ? language : "en";
 	}
 }