go back to strings to accept arbitrary culture codes

src/Umbraco.Web.UI.Client/src/apps/app/app.element.ts

@@ -30,9 +30,11 @@ export class UmbAppElement extends UmbLitElement {
 	 *
 	 * @attr
 	 * @remarks This is the default culture to use for localization, not the current culture.
+	 * @example "en-us"
+	 * @example "en"
 	 */
 	@property({ type: String, attribute: 'default-culture' })
-	culture: Intl.BCP47LanguageTag | Intl.Locale = 'en-us';
+	culture: string = 'en-us';
 
 	/**
 	 * The base path of the backoffice.

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

@@ -62,9 +62,8 @@ export class UmbTranslationRegistry {
 	 * 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());
+	loadLanguage(locale: string) {
+		this.#currentLanguage.next(locale.toLowerCase());
 	}
 
 	#addOrUpdateDictionary(