It says that it is not possible to book an extra seat in the Premium Economy and Business cabins https://wwws.airfrance.de/en/information/passagers/forte-corpulence, in Economy it's ok. But it can changes according to  different companies 

> Book two seats Please contact our Saphir service so that they can
> assess your situation, check if a seat belt extension is necessary,
> and complete your booking with you. 
> 
> If you are traveling in the Economy cabin on one of our long-haul
> flights, and the width of the seat is not sufficient for you, Saphir
> will offer you a second seat next to yours. This second seat will
> provide you with greater comfort, while ensuring your safety with a
> seat belt extension. 
> 
> ...
> 
> Please note: in the Premium Economy and Business cabins, it is not
> possible to book an extra seat with a seat belt extension, due to the
> seat configuration and static armrests.
> 
> In the Economy cabin: passengers cannot book two seats side by side
> and for the same person using our online booking system.