app.component.html

<header>
    <button (click)="lang.next('fr')">FR</button>
    <button (click)="lang.next('en')">EN</button>
</header>

app.component.ts

@Component({
    selector: 'i-app',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {

    lang = new Subject<string>();
    lang$: Observable<string>;

    constructor(private readonly translate: TranslateService,
        private readonly store: Store<fromRoot.State>) {
        this.translate.setDefaultLang('en');

        this.lang$ = this.store.select(fromRoot.getCurrentLanguage);
    }

    ngOnInit() {
        this.lang
            .startWith(this.translate.getBrowserLang())
            .subscribe(lang => this.store.dispatch(new language.Set(lang)));

        this.lang$.subscribe(lang => this.translate.use(lang));
    }

}