A med school interview is an opportunity for an admission committee to evaluate how an applicant thinks and would interact with people (more specifically, patients), and also get a glimpse of the applicant's character. I don't think being talkative is always a good thing because I believe the quality of your talk (like your reasoning, insight) is more important than the quantity. But, you still have to demonstrate decent communiaction skills because that's a pivotal quality in a physician, sort of a people's person🙂.
Usually, applicants don't have the problem of being not talkative during interviews because many times interviewers will bombard them with questions or sometimes they'd rather talk about their life stories than the applicants'. But, in most cases, your interviewers will guide you through their questions and try their best to make the whole interview process as pleasant and meaningful as possible.
You may not be fully fluent in English, but don't let it stop you. What matters during an interview is that you are open and can express your thoughts clearly and articulately, not your pronunciation or imperfect choice of words due to your immigration background. So, don't be pressured to be someone whom you are not. Just try to have a fun, genuine, meaningful conversation with your interviewers. Good luck.