ngneat/transloco/lobby

@avifox spectator is not necessary, but it really makes everything easier. My examples are below.
For missing keys, I agree with @shaharkazaz , if you want to check for missing keys, just run transloco-keys-manager find and look for the missing and unused keys. If you wanted it automated, maybe there is a way to parse the output and make it a pass/fail step in your build process.

i pulled out my setLanguage promise into a function that i can call:

  /** setting the language does not happen immediately. It must first load,
   * then change detection must fire.
   * This method will set the lang, and wait for it to load and detect changes. */
  async function setLang(lang: string) {
    await new Promise(resolve => {
      translocoService.events$.pipe(
        filter((t: TranslocoEvents) => t.type === 'translationLoadSuccess')
      ).subscribe(t => resolve());
      translocoService.setActiveLang(lang);
    }).then(() => spectator.detectChanges());
  }

here is my beforeEach:

beforeEach(() => {
    // tests were written for the case that spanish is the default language
    TranslocoConfigProvider.useValue.defaultLang = 'es';
    spectator = createSpectator();
    translocoService = spectator.inject(TranslocoService);
    // set lang here so we can make sure it is loaded before running tests
    setLang('es');
  });

make sure to also use the TranslocoTestingModule, here's what I used, borrowed from Transloco:

export const languageMap = {
  en,
  es,
  'i18nTest/en': admin,
  'i18nTest/es': adminSpanish,
  'lazyPage/en': lazy,
  'lazyPage/es': lazySpanish,
  'mf/en': transpilersMessageFormat,
  'mf/es': transpilersMessageFormatSpanish
};

export function getTranslocoTestModule(config: Partial<TranslocoConfig> = {}) {
  return TranslocoTestingModule.withLangs(
    languageMap,
    {
      availableLangs: ['es', 'en'],
      defaultLang: 'es',
      fallbackLang: 'en',
      ...config
    }
  );
}