I am studying 3 at the same time and speak English my native language and also Spanish. It definitely slows down your progress. It depends on what your goals are. If you are learning for fun, then it doesn't matter, but if you need one of the languages for work or a specific purpose, you should stick to that one. I am learning Portuguese , Italian, and French together. Portuguese is very confusing with Italian. It is not a good idea for those two. French is different from other Romance language and I do not confuse it with the others. I agree that French and English are very different and would not be confusing. However, as I said before, studying the 2 together will slow down your progress in each.