use dash (-) instead of underscore (_) for backward compatibility

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

@@ -46,7 +46,7 @@ export class UmbBackofficeElement extends UmbLitElement {
 		this.consumeContext(UMB_AUTH, (auth) => {
 			this.observe(auth.currentUser, (user) => {
 				if (user) {
-					const languageIsoCode = user.languageIsoCode ?? 'en_us';
+					const languageIsoCode = user.languageIsoCode ?? 'en-us';
 					localizationContext.setLanguage(languageIsoCode);
 				}
 			});

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

@@ -18,8 +18,8 @@ export class UmbLocalizationContext {
 	 * Set a new language which will query the manifests for translations and register them.
 	 * Eventually it will update all codes visible on the screen.
 	 *
-	 * @param languageIsoCode The language to use (example: 'en_us')
-	 * @param fallbackLanguageIsoCode The fallback language to use (example: 'en_us', default: 'en_us')
+	 * @param languageIsoCode The language to use (example: 'en-us')
+	 * @param fallbackLanguageIsoCode The fallback language to use (example: 'en-us', default: 'en-us')
 	 */
 	setLanguage(languageIsoCode: string, fallbackLanguageIsoCode?: string) {
 		this.#translationRegistry.register(languageIsoCode, fallbackLanguageIsoCode);

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

@@ -18,7 +18,7 @@ export class UmbTranslationRegistry {
 		return this.#innerDictionary.asObservable();
 	}
 
-	register(userCulture: string, fallbackCulture = 'en_us') {
+	register(userCulture: string, fallbackCulture = 'en-us') {
 		userCulture = userCulture.toLowerCase();
 		fallbackCulture = fallbackCulture.toLowerCase();
 

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

@@ -6,11 +6,11 @@ const translationManifests: Array<ManifestTranslations> = [
 		type: 'translations',
 		alias: 'Umb.Translations.En_US',
 		weight: -100,
-		name: 'English (UK)',
+		name: 'English (US)',
 		meta: {
-			culture: 'en_us',
+			culture: 'en-us',
 		},
-		loader: () => import('../../../assets/lang/en_us.js')
+		loader: () => import('../../../assets/lang/en-us.js'),
 	},
 ];
 

src/Umbraco.Web.UI.Client/storybook/stories/extending/localization.mdx

@@ -24,9 +24,9 @@ The manifest can be added through the `umbraco-package.json` file like this:
 			"alias": "MyPackage.Lang.EnUS",
 			"name": "English (United States)",
 			"meta": {
-				"culture": "en_us"
+				"culture": "en-us"
 			},
-			"js": "/App_Plugins/MyPackage/lang/en_us.js"
+			"js": "/App_Plugins/MyPackage/lang/en-us.js"
 		}
 	]
 }
@@ -36,7 +36,7 @@ If you do not have many translations, you can also choose to include them direct
 
 ```json
 "meta": {
-	"culture": "en_us",
+	"culture": "en-us",
 	"translations": {
 		"section": {
 			"key1": "value1",