The groups of french-speaking players I can think of (I'm probably missing some...):
La compagnie Caradoc, a huge group (100+) of players coming mostly from France.
They play mercenaries/settlers style.
You can find them on , on , and you can find more information as well.
Les haquebutes de Talabeim. A group of 20-30 (?) players coming mostly from France.
They play lansquenets in a setting inspired by Warhammer Fantasy's empire.
You can find them on .
L'Ost noir, which I'm part of. A group of 30+ players coming mostly from Switzerland.
Our play is Chaos-oriented, with a main focus on Slaanesh worship.
You can find us on .
Equinox, another group of 20-30 (?) Swiss people.
They play Elves and other kinds of wooden-type characters.
You can find them on as well.
La phalange noire. A huge group from Swiss players as well (over here they're over 60 players during LARPs). This year will be their first time in Mythodea, and I think they have around 30 (?) people coming.
They play a kind of religious army/crusade.
I found no easy way to reach them, but if you contact on Facebook he should be able to help you.
There's a couple smaller groups as well, but I can't remember their name or contact information right now.
As for contacting NPC organizers, I don't know, you should try the general forum (Allgemeines).