Why does not work function in Angular 2 Typescript is called from template?

I have a following tag:

<tag-input hideForm="true" [onRemoving]="onRemoving"></tag-input>

Where [onRemoving]="onRemoving" is function is called by attempts to delete value.

It looks like:

public onRemoving(tag: any): Observable<any> {

    let del = '';

    this.translate.get('confirmation_text').subscribe((res: string) => {
      del = res['confirmation_text'];
    });
}

Problem is in this code:

this.translate.get('confirmation_text').subscribe()...

I get an error:

TypeError: Cannot read property 'get' of undefined

I think cause is in area of visibility of this context

Constructor:

constructor(private translate: TranslateService) {
}