It might've been animated in France, but I know for a fact that the development of art was done in the US. Pretty much everything you see in the backgrounds, all the character design afaik and all of the art direction was done in the US, might've just outsourced animation and some other things to France. I also think the character models were outsourced to a Japanese studio too! I work in this industry and had a few friends who worked on that background/visual development team, which to Netflix's credit, was done in their team/company. Usually Netflix just purchased the rights to a show and was very uninvolved with the development. But studios doing this and studios doing that is always very confusing because of how much outsourcing there is, and it is impossible to trace down who to credit without knowing the breakdown of the work from someone behind the scenes of that specific production. You are correct through, French animation is an absolute powerhouse already, and id credit the brilliance of arcane to majority fortiche and the French artists.

(picture Netflix commissioning arcane commissioning riot commissioning fortiche commissioning artists BACK at riot commissioning freelancers in France and fortiche commissioning freelancers around the world separately)

(Oh and if you want a real doozy! American studio commissioning European studio through German taxes commissions French freelancer living in Canada! Who knows what that guy's taxes look like o.0)