I would suggest that your English will need to be fluent in order to be able to cope with the FI course. When I did mine, a lot of emphasis was put on the use of correct terminology and on the clarity of everything you say. You will have to do all this whilst flying the aircraft to a high standard. Also, you will need to demonstrate briefings and give short lectures.
Judging by your post, your English seems pretty good to me. You will have to go on an acceptance flight with and FIC instructor and he/she perhaps will be able to assess your level of English and can advise as to how this will affect your chances of success.