Hello everybody !
It is possible to have Arx Fatalis in other languages on GOG ? Specially in french since it's a french game ...

Gog's only offering games in English now. After the beta period is over, they'll probably try to get other language versions.

Bizarrely, Arx Fatalis was designed and written in English, so the English localisation is actually the original one. Arkane did this because they decided the English-speaking market was the most important one to target. More about that here.
GOG will be adding support for French, German and Polish, presumably after the site comes out of beta. At this point in time, however, all releases are in English (although some games include multiple languages).

It's interesting how my favorite RPGs of late all come from european developers, most of them meant for non-english speakers (all three Gothics, The Witcher, Arx Fatalis, others). I'm also a great fan of the The Dark Eye trilogy and look forward to playing Drakensang...

Thanks for answers ! I was not aware of their languages issues ...
But I'm not really surprised, of course the English-speaking market is the first one ...
That said, in the case of other french games plan for the world market, the french version was sometimes obviously the best ... I think particularly to "Beyond good and evil" ... But I know Arkane is not Ubisoft-Montpellier ....
Whatever, my english is not fluent enough to play an entire no-subtitled game in english ... I read english without problem but I can't follow all-audio speeches ...
I will wait for french version ...

After 10 years the situation is the same and Gog only offers English for the majority of their classics titles. Such a infuriating policy for non english speakers

It's not necessarily a policy, often they need to aquire the rights for the localizations separately because they were made by local distributors rather than the original publisher and like many games it's often a matter of finding who has them in the first place.
And this assuming once they untangled the mess they have the relevant files in archive or that GoG can find a copy of the localized version somehow, and assuming that the localized version doesn't have weird differences like localization specific patches.