Hassle from teenage gangs ?
Errr... Sometimes I guess, like everywhere... No gun fights if that's what you're thinking about

Paris is quite a calm city even if there are obviously some "sensitive" blocks...
But for skating the last 3 years in Paris and its suburbs, I never have any problem.
I don't understand your second question... What's difficult if you don't speak french ? Skating ? If that's the point, I don't think it's a problem,you can easily find someone who can speak a little english.
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