Lots of technical improvements

src/app/service/language.service.spec.ts

@@ -0,0 +1,43 @@
+import { TestBed } from '@angular/core/testing';
+import { TranslateService } from '@ngx-translate/core';
+import { LanguageService } from './language.service';
+import { LocalStoreConstants } from '../constants/LocalStoreConstants';
+
+describe('LanguageService', () => {
+  let translateSpy: jasmine.SpyObj<TranslateService>;
+
+  function createService(): LanguageService {
+    translateSpy = jasmine.createSpyObj<TranslateService>('TranslateService', ['use']);
+    TestBed.configureTestingModule({
+      providers: [{ provide: TranslateService, useValue: translateSpy }],
+    });
+    return TestBed.inject(LanguageService);
+  }
+
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  afterEach(() => {
+    localStorage.clear();
+  });
+
+  it('uses the stored language as the initial value', () => {
+    localStorage.setItem(LocalStoreConstants.LANGUAGE_KEY, 'de');
+    const service = createService();
+
+    expect(service.lang()).toBe('de');
+    expect(translateSpy.use).toHaveBeenCalledWith('de');
+  });
+
+  it('use() updates the signal, the translate service and localStorage', () => {
+    localStorage.setItem(LocalStoreConstants.LANGUAGE_KEY, 'en');
+    const service = createService();
+
+    service.use('de');
+
+    expect(service.lang()).toBe('de');
+    expect(translateSpy.use).toHaveBeenCalledWith('de');
+    expect(localStorage.getItem(LocalStoreConstants.LANGUAGE_KEY)).toBe('de');
+  });
+});