Usually Interview is conducted in English. It also depends on subject for which interview is being held. The answer should be in the same language as of the question. It also depends on how comfortable one is in particular language. The purpose of Interview is not just to check the English language rather check one's approach to the topic under discussion. So, One can use switching method from English to Urdu or Urdu to English. What matters most is to convey one's thoughts / opinion as clear as possible.

__________________
There are certain questions that must never be asked. Of anyone. Even oneself.