[skydhash]

There is no Etage 0. People will just confusingly look at you if you ever said that. And the "Je ne sais pas' is called "Sous-sol"

[gwhl]

In a lot of elevators rez-de-chaussée is labelled 0. We just don't use that when communicating

[scraptor]

You can have multiple basement levels, which get numbered again. 1er Sousl-sol, 2eme Sous-sol etc. The ground floor might not be explicitly labeled as zero but the number between "one up" and "one down" is still zero.