I think it is important to talk about this. I'm a Mandarin speaker and I normally don't answer questions that asked in traditional Chinese/Cantonese, because it is very likely I don't have the best answer.
However, I have seen people answer questions in traditional Chinese/Cantonese when the questions are asked in English/Mandarin/simplified Chinese. I think in this case, the answerer should let the poster know that the answer is in traditional Chinese/Cantonese, especially the poster doesn't show a high level of competence in Chinese.
Maybe we should guide the asker to state whether they are learning Mandarin or Cantonese too. What do you think?