Do you speak fluent English?

The interviewer simply wants to know more about you and your background, and they will likely be able to pick up on this during your interview. Be honest with the interviewer. If you speak fluent English, say, "yes, I do!" If you do not speak fluent English, that is okay. Tell the interviewer that you are not fluent, and expand on the level of proficiency that you have. Now is also a great time to mention any other languages that you speak fluently.