Everyday I walk to/from the Metro and my apartment as well as my office. I have been observing people riding motorcycles, motor-scooters (like Vespas), and bicycles. There are lots of cars, but lots of these smaller vehicles as well.
I would say the vast majority of motorcycle/motor-scooter riders wear helmets. But, it is not 100%. I do not know if France has a law about helmets.
I would also say the vast majority of bicycle riders do not wear helmets. Again, it is not 100%. There is the occasional cyclist with a helmet, but it is clearly the exception.
There are separate bicycle lanes on some of the larger sidewalks, but most of the bicycle traffic is in the street with everything else. Of course, it is the middle of a large city and it is never high speed. But, the ethic of bicycle helmets has just not occurred here.
One anecdote — I was walking along an avenue near my apartment when I saw a motorcycle rider have to stop in an intersection to be able to turn left. As it was a wait, their left foot came down to stabilize the cycle. I only saw it in profile, but the leg ended in a stiletto high heel. I do not think I’ve ever seen someone riding in stilettos. She must have been riding to work.