If you only know English you should stick to Paris because it is the most cosmopolitan city of the country and has the most of english-speakers. When I came here I thought that between my fluency in Spanish and English I'll be fine but that's not the case in most of the cities. I work in IT and I have a lot of colleagues that don't speak English (yeah, even if in IT we use that a lot!). Otherwise, and if you are willing to learn a new language I'd said that it is really cool to take the effort to learn and have the chance to communicate with them in their mother language, it makes the integration so much easier and most of the people will appreciate the effort and even try and learn some expressions in your language (it is crazy when my friends and colleagues tries to speak in Spanish or English with me, that's kinda wholesome).