Knowing basic French is quite important when studying in France, even if you do not take courses. Researchers here are of course at least decent in English, but other personnel such as administrative staff are not always able to communicate in English. However, I am by no means fluent in French and I still got along fine, so you don't have to be a native to get around.
There are French courses at UPMC for exchange students which you can take for free. I did so last time and found them very helpful, but this semester I just didn't have time for it. If you have the time and are not fluent in French, I would suggest taking a course. Before the start of the courses there is a test to determine your level.
Other than language barriers, I've never had much problems adjusting to Paris culture. It really isn't all that different from Stockholm, other than the weather. People are quite reserved so if you want to get to know people it's best to put yourself out there, just like at home.