Localization: Fixes an issue where the fallback localization was not available after changing DefaultUILanguage (fixes #20216) (#20230)

fix: always load the default/fallback language code so it is available

src/Umbraco.Web.UI.Client/src/packages/core/localization/registry/localization.registry.ts

@@ -118,6 +118,9 @@ export class UmbLocalizationRegistry {
 			)
 			// Subscribe to the observable to trigger the loading of translations
 			.subscribe();
+
+		// Always register the fallback language (en) to ensure there is always at least one language available
+		this.loadLanguage(UMB_DEFAULT_LOCALIZATION_CULTURE);
 	}
 
 	#loadExtension = async (extension: ManifestLocalization) => {