The legalities of a Humanist Ceremony ?

France is very clear on the legalities of marriage.
There is no such thing as a registrar in France. The only person who is legally allowed to perform a marriage ceremony is an elected member of a council (e.g. The Maire or his/her adjoints) 

Any other type of ceremony , religious or non-religious is considered symbolic.
Even a catholic ceremony in a church has no legal validity in France. 

“( Le mariage religieux y est possible mais n’a aucune valeur légale depuis 1791.)”

The only condition for a symbolic ceremony ( especially a Church one) is that the civil ceremony is performed first.(I don’t think this applies any more).

“L’article 433-21 du code pénal punit de six mois d’emprisonnement et de 7 500 euros d’amende tout ministre d’un culte qui procède, de manière habituelle, aux cérémonies religieuses de mariage sans que lui ait été justifié l’acte de mariage préalablement reçu par les officiers de l’état civil.”

If you want a legal solemnizing by a humanist celebrant you must perform the ceremony in your own country. 

Most couples nowadays who are looking for a non-religious but meaningful ceremony opt to do the legal paperwork first quietly in the most convenient country with their witnesses. They then have a  large “symbolic” ceremony surrounded by their family and friends in any country of their choice.

Although a non-religious, humanist blessing is not a legally recognized marriage ceremony, it is a perfect solution if you are unable to meet the residency and legal requirements in the country where you would like to celebrate your wedding.

I do many French ceremonies immediately after the civil one at the Mairie.

I hope I haven’t jangled your brains with all this info.
If you have any questions please don’t hesitate to ask.