Flow of thought should be present if you don’t have words to express yourself then it is better to take an interview course or public speaking course. If you cannot express yourself then there is no point in attending an interview. But if you have adequate technical knowledge and facing difficulty to express it in English then ask permission of the interviewer to express your opinion or interview in your native language and then show him sufficient documents that you are trying to develop command in the language. Attend a tutorial or classes because it can have a significant impact in future.