feat(core-app-api): support no en languages

Signed-off-by: JounQin <admin@1stg.me>

.changeset/tasty-yaks-kiss.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-app-api': patch
+---
+
+feat: support no en languages

packages/core-app-api/src/apis/implementations/AppLanguageApi/AppLanguageSelector.test.ts

@@ -144,11 +144,26 @@ describe('AppLanguageSelector', () => {
       AppLanguageSelector.createWithStorage({
         availableLanguages: ['de'],
       }),
-    ).toThrow("Supported languages must include 'en'");
+    ).toThrow(
+      "Initial language must be one of the supported languages, got 'en'",
+    );
 
     const selector = AppLanguageSelector.createWithStorage(baseOptions);
     expect(() => selector.setLanguage('sv')).toThrow(
       "Failed to change language to 'sv', available languages are 'en', 'de'",
     );
   });
+
+  it('should support no en languages', () => {
+    const selector = AppLanguageSelector.createWithStorage({
+      availableLanguages: ['de'],
+      defaultLanguage: 'de',
+    });
+
+    expect(selector.getLanguage()).toEqual({ language: 'de' });
+
+    expect(() => selector.setLanguage('en')).toThrow(
+      "Failed to change language to 'en', available languages are 'de'",
+    );
+  });
 });

packages/core-app-api/src/apis/implementations/AppLanguageApi/AppLanguageSelector.ts

@@ -44,11 +44,9 @@ export class AppLanguageSelector implements AppLanguageApi {
         )}'`,
       );
     }
-    if (!languages.includes(DEFAULT_LANGUAGE)) {
-      throw new Error(`Supported languages must include '${DEFAULT_LANGUAGE}'`);
-    }
 
     const initialLanguage = options?.defaultLanguage ?? DEFAULT_LANGUAGE;
+
     if (!languages.includes(initialLanguage)) {
       throw new Error(
         `Initial language must be one of the supported languages, got '${initialLanguage}'`,