[retrac]

Such laws do not supersede the constitutions of its sovereign member nations. If a member nation's constitution requires something that would place them in violation a treaty agreement, then it would have to fix its constitution or be in violation. The treaty itself doesn't change municipal law, though. (Unless it's a country where it does, anyway. But France is not one of them.)

> unless they take over the execution of the prison sentence against the wanted person

Yes, that bit. France still cannot extradite its citizens, but it can meet its obligations to its partners by prosecuting and incarcerating someone within France.