add comments and requirements for language

src/Umbraco.Web.UI.Client/src/libs/localization-api/registry/translation.registry.ts

@@ -57,8 +57,13 @@ export class UmbTranslationRegistry {
 		);
 	}
 
-	loadLanguage(userCulture: string) {
-		this.#currentLanguage.next(userCulture.toLowerCase());
+	/**
+	 * Load a language from the extension registry.
+	 * @param locale The locale to load.
+	 */
+	loadLanguage(locale: Intl.BCP47LanguageTag | Intl.Locale) {
+		locale = new Intl.Locale(locale);
+		this.#currentLanguage.next(`${locale.language}-${locale.region}`.toLowerCase());
 	}
 
 	#addOrUpdateDictionary(