I'm sure this question is with reference to India and its sub-continents where English is assumed to be the first language while facing an interview. Otherwise in other countries like China, Spain, and France etc, it really matters if you know English or not. Any employer will look into what skills they are looking for and what is level candidate of a candidate in that skill and for sure how confident that candidate feels about the job he is going to do.
When appearing for an interview, the ideal combination of fluent English, confidence, body language, facial expressions, and technical knowledge is important. No employer would recruit people who look depressed or distressed. English is what we learn throughout our education system, but confidence can't be taught anywhere
In normal day to day activity, we speak English fluently and with confidence. When it comes in situations like interview, we get into unknown stress. When you're attending an interview means you're sharing your valuable time with your expected employer. You loose your concentration when you start thinking of other situations just before interview.
Be what you are; if you act, you will be caught. Interviewers are experts and you don't act too smart or try to fool them. What happens at interview time: