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.
Interviews in Indian Scenario:
Now let's take Indian scenario, even though we are expert in our area of expertise but if we are not capable enough to communicate our thoughts using English then our interviewer thinks we are not capable enough and finally get a rejection. Keeping the same facts around, usually candidates with poor communication feel nervous and usually do not do so well in their interviews.
Bottom line, if you are facing an interview in India or its sub-continents like Pakistan, Sri Lanka, Nepal etc then you must have great English fluency to crack your interviews in good organizations. For sure, if you have great communication then your confidence will boost automatically.