Измените ваш код
public class InputFragment extends Fragment implements View.OnClickListener
translate_button.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View InputFragmentView)
{
Log.d("Test", "onClickListener ist gestartet");
Toast.makeText(getActivity().getApplicationContext(), "Test", Toast.LENGTH_LONG).show();
saveInString();
}
});