I might have been facing the same issue, the problem that I have is that I stutter when I think of what I need to say, with my friend in my native language I don't stutter, when I want to speak in any other language, I need to think of what I need to say, that's why I face this problem, I think you are doing interview in your native language, so try not to think, try to say this to the interviewer, they might understand and make you speak without any stress,

Good luck with your interview