app-comp.component.ts

@Component({
  selector: 'app-comp',
  templateUrl: './app-comp.component.html',
  providers: [
    {
      provide: TRANSLOCO_LOADING_TEMPLATE,
      useValue: '<p>Translation is loading...</p>'
    }
  ]
})
export class AppComp {}

some-template.html

<ng-container *transloco="let t; loadingTpl: loading">
  <h1>{{ t('title') }}</h1>
</ng-container>

<ng-template #loading>
  <h1>Loading...</h1>
</ng-template>