Finish learning English first. Personally I have been learning Japanese for the past three years and recently I started taking a college course on Spanish. But I mix these two languages up a lot mostly because I know more Japanese than Spanish. They also have the same vowel sounds. When I want to say something in Spanish but don't know how to, I use the Japanese word instead.  Its a bad habit however, its not uncommon when learning more than one language. Unless you truly feel like you can do it. I highly advice you to not do it. I also want to learn French because I took it in high school and loved it but after high school, I forgot most of it. I'm going to learn French after I'm fluent in Japanese. After this class I'll probably study Spanish in my free time but I'm mostly going to focus on my Japanese. kudos to you if you decide to take on French and English.  

Am'r, depends on time you dedicate to English. An old friend of mine had French, Latin and Greek in high-school (may be Enlgish too, I don't remember). Latin, Greek. French, Italian, German, Modern Greek in unieversity. Other European languages later. Of course he is fluent in some of these langauges.

But if  French means that you'll stop working on your English... then your English will get worse. Not because you studied French, but because you stopped doing English.

A good thing about having experience with more than one language is that you understand more about how languages work. This experience helps somehow. If you start learning French, keep reading and communicating in English.

I think, it is a very individual matter. For someone -as jimmy said - it is difficult to learn two languages at once. But for another, it can be very helpfull. I personally have good experience with it. In many schools the children learn two or three languages at once. Especially in your case (upper-intermediate level) i think, one can start with second language without problem.

I exactly have the same case of you expect for I am learning spanish instead of French, You really mentioned a good topic,I wish people who have experienced it would help us.

I will consider your advice , Jimmy 
Thank you very much , I think I won't start to learn French until i'm fluent in English