In short, often happens that a quite simple question got a quite fast answer but answer is not applicable since answerer did not read/understood the question properly. I understand the language could be issue too, since not all users are proficient in English. Should we down-vote the answer (but I believe such mistakes are made in good will so unfair to down-vote) or just comment under explaining the mistake, wrong information?
If you think it's completely wrong, the first step should be to point out where the answerer went awry. Leave a comment. This is necessary (in my mind) because you might be wrong and might be misunderstanding the significance of the answer. If you leave a comment, the problem can be addressed, or countered and you can be notified.
Then vote accordingly. If you've left a comment and you have left a way for the poster to reprise themselves, this isn't unfair. You'll have the opportunity to rectify your vote.
If the quality is so awful that it's beyond any meaningful editing, that's the point where you start flagging and we start deleting.