A good interview should be able to render interviewee freedom to express oneself so as to enable the interviewer reach the objectives of the interview. This implies that the language being used must be understandable to the interviewee save that the interview's objective is to obtain a person who masters English grammar. And for showing that you are competent and confident, don't answer any question that you'ven't well understood, better demand clarification in other words or you'll sound stupid attempting answering what you don't know.