Added tests to reproduce #1388 - but everything seems to be ok.

projects/ngx-translate/src/lib/translate.pipe-standalone.spec.ts

@@ -40,6 +40,23 @@ class FakeChangeDetectorRef extends ChangeDetectorRef {
   }
 }
 
+@Injectable()
+@Component({
+  selector: 'lib-hmx-app',
+  standalone: true,
+  imports: [TranslatePipe],
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  template: `{{'default' | translate}}`
+})
+class AppTranslationIdDefaultComponent {
+  viewContainerRef: ViewContainerRef;
+
+  constructor(viewContainerRef: ViewContainerRef) {
+    this.viewContainerRef = viewContainerRef;
+  }
+}
+
+
 @Injectable()
 @Component({
   selector: 'lib-hmx-app',
@@ -123,6 +140,15 @@ describe('TranslatePipe (standalone)', () => {
     expect(translatePipe.transform('TEST')).toEqual("This is a test");
   });
 
+  it("should translate 'default'", () =>
+  {
+    prepare();
+    translate.setTranslation("en", {"default": "This is the default message"});
+    translate.use("en");
+
+    expect(translatePipe.transform("default")).toEqual("This is the default message");
+  });
+
   it('should call markForChanges when it translates a string', () => {
     prepare();
     translate.setTranslation('en', {"TEST": "This is a test"});
@@ -374,5 +400,20 @@ describe('TranslatePipe (standalone)', () => {
       fixture.detectChanges();
       expect(fixture.debugElement.nativeElement.innerHTML).toEqual("This is a test");
     });
+
+
+    it("translate text with 'default' message id" , () => {
+      prepare();
+
+      const fixture = TestBed.createComponent(AppTranslationIdDefaultComponent);
+
+      translate.setTranslation("en", {"default": "This is some default text"});
+
+      fixture.detectChanges();
+      expect(fixture.debugElement.nativeElement.innerHTML).toEqual("default");
+      translate.setDefaultLang('en');
+      fixture.detectChanges();
+      expect(fixture.debugElement.nativeElement.innerHTML).toEqual("This is some default text");
+    });
   });
 });

projects/ngx-translate/src/lib/translate.service.spec.ts

@@ -165,6 +165,18 @@ describe("TranslateService", () =>
     });
   });
 
+  it("translates text using 'default' translation Id", () =>
+  {
+    translations = {"default": "Default text"};
+    translate.use("en");
+
+    // this will request the translation from the backend because we use a static files loader for TranslateService
+    translate.get("default").subscribe((res: Translation) =>
+    {
+      expect(res).toEqual("Default text");
+    });
+  });
+
   it("should be able to get an array translations", () =>
   {
     translations = {"TEST": "This is a test", "TEST2": "This is another test2"};
@@ -702,6 +714,16 @@ describe("TranslateService", () =>
       });
       translate.set("TEST", "This is a test");
     });
+
+
+    it("should translate 'default'", () =>
+    {
+      translate.setTranslation("en", {"default": "This is the default message"});
+      translate.use("en");
+
+      expect(translate.instant("default")).toEqual("This is the default message");
+    });
+
   });
 
   it("should trigger an event when the lang changes", () =>