Note that french people saying "inshallah" say it as a joke (they use this word for fun effect), which isn't the same as really adopting this word. As for arab-derived slang, i know a lot of parents who absolutely forbid their kids to use those words, and the kids only do it to rebel, or there again to sound "special". But they'll be extra careful ( and will be able to) stop speaking in this manner in a professional context.
Which is to say, there's still a HUGE divide in the population, and the fact that the "french" part is using a few words from the other part shouldn't fool you into thinking that immigration is going well.
Don't be surprised by the results in the upcoming EU poll.