core-app-api: add support for explicit configuration of default language

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

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

@@ -25,6 +25,7 @@ export const DEFAULT_LANGUAGE = 'en';
 
 /** @alpha */
 export interface AppLanguageSelectorOptions {
+  defaultLanguage?: string;
   availableLanguages?: string[];
 }
 
@@ -47,7 +48,14 @@ export class AppLanguageSelector implements AppLanguageApi {
       throw new Error(`Supported languages must include '${DEFAULT_LANGUAGE}'`);
     }
 
-    return new AppLanguageSelector(languages);
+    const initialLanguage = options?.defaultLanguage ?? DEFAULT_LANGUAGE;
+    if (!languages.includes(initialLanguage)) {
+      throw new Error(
+        `Initial language must be one of the supported languages, got '${initialLanguage}'`,
+      );
+    }
+
+    return new AppLanguageSelector(languages, initialLanguage);
   }
 
   static createWithStorage(options?: AppLanguageSelectorOptions) {
@@ -85,9 +93,9 @@ export class AppLanguageSelector implements AppLanguageApi {
   #language: string;
   #subject: BehaviorSubject<{ language: string }>;
 
-  private constructor(languages: string[]) {
+  private constructor(languages: string[], initialLanguage: string) {
     this.#languages = languages;
-    this.#language = languages[0];
+    this.#language = initialLanguage;
     this.#subject = new BehaviorSubject<{ language: string }>({
       language: this.#language,
     });

packages/core-app-api/src/apis/implementations/TranslationApi/I18nextTranslationApi.test.ts

@@ -193,6 +193,29 @@ describe('I18nextTranslationApi', () => {
     expect(snapshot.t('foo')).toBe('OtherFoo');
   });
 
+  it('should allow initial language to not be the default one', async () => {
+    const languageApi = AppLanguageSelector.create({
+      defaultLanguage: 'sv',
+      availableLanguages: ['en', 'sv'],
+    });
+    const translationApi = I18nextTranslationApi.create({
+      languageApi,
+      resources: [
+        createTranslationResource({
+          ref: plainRef,
+          translations: {
+            sv: () => Promise.resolve({ default: { foo: 'Föö' } }),
+          },
+        }),
+      ],
+    });
+
+    const snapshot = assertReady(
+      await waitForNext(translationApi.translation$(plainRef), s => s.ready),
+    );
+    expect(snapshot.t('foo')).toBe('Föö');
+  });
+
   it('should prefer the last loaded resource', async () => {
     const languageApi = AppLanguageSelector.create({
       availableLanguages: ['en', 'sv'],

packages/core-app-api/src/apis/implementations/TranslationApi/I18nextTranslationApi.ts

@@ -155,6 +155,11 @@ export class I18nextTranslationApi implements TranslationApi {
 
     i18n.init();
 
+    const { language: initialLanguage } = options.languageApi.getLanguage();
+    if (initialLanguage !== DEFAULT_LANGUAGE) {
+      i18n.changeLanguage(initialLanguage);
+    }
+
     const loader = new ResourceLoader(loaded => {
       i18n.addResourceBundle(
         loaded.language,

packages/core-app-api/src/app/AppManager.tsx

@@ -182,6 +182,7 @@ export class AppManager implements BackstageApp {
     this.bindRoutes = options.bindRoutes;
     this.apiFactoryRegistry = new ApiFactoryRegistry();
     this.appLanguageApi = AppLanguageSelector.createWithStorage({
+      defaultLanguage: options.__experimentalTranslations?.defaultLanguage,
       availableLanguages:
         options.__experimentalTranslations?.availableLanguages,
     });

packages/core-app-api/src/app/types.ts

@@ -284,6 +284,7 @@ export type AppOptions = {
   bindRoutes?(context: { bind: AppRouteBinder }): void;
 
   __experimentalTranslations?: {
+    defaultLanguage?: string;
     availableLanguages?: string[];
     resources?: Array<TranslationMessages | TranslationResource>;
   };