only one default language

src/Umbraco.Web.UI.Client/src/core/mocks/data/languages.data.ts

@@ -28,7 +28,7 @@ class UmbLanguagesData extends UmbData<UmbLanguageStoreItemType> {
 			}
 		});
 
-		return saveItems;
+		return this.data;
 	}
 
 	delete(keys: Array<string>) {
@@ -50,6 +50,14 @@ class UmbLanguagesData extends UmbData<UmbLanguageStoreItemType> {
 		const itemKeys = Object.keys(item);
 		const newItem = {};
 
+		if (updateItem.isDefault) {
+			this.data.forEach((item) => {
+				if (updateItem !== item) {
+					item.isDefault = false;
+				}
+			});
+		}
+
 		for (const [key] of Object.entries(updateItem)) {
 			if (itemKeys.indexOf(key) !== -1) {
 				// eslint-disable-next-line @typescript-eslint/ban-ts-comment