A few years ago I started my flight training in an English speaking country. I could manage written English quite well while expressing myself and my ability to understand someone speaking was ok but far from top notch. Apart from some "say agains" on the radio in the beginning of my training it wasn't really any problem. You will improve your English quite fast if you use the language on an everyday basis. Today, I am better off to describe a technical term related to aviation in English as opposed to my native language.
Judging by your post your written English is good. I think that you will have no problem with your language. After a month or two in an English speaking country you will probably have improved your oral English considerably. By the way, many of the radio speakers, especially in the US, are trying to hit some world record in speaking as fast as possible and preferably in an inarticulated way!
However, understanding French...
