Yup, French accents are incredibly different from region to region. I think the cajun accent will be the biggest hurdle for any actor cast in the role.
I like Gaspard Ulliel (and I'd rather they had cast him instead of CT for the Fox film...though I know, he has a lot less "star" power than CT), but for Marvel, they would probably need to cast someone in their twenties if they want to build a franchise for the X-men over a period of, say, 10 years or more.