No, to dive in France, you need either an FFESSM/CMAS cert. For FFESSM, you're either N2 and limited to 20m or you're N3 which is 60m. These certs are how most dive operators here run things. Instructor for x y or z agency? N2. Because you don't have the deep air and decompression cert. And yes, this is law, part of the Code du Sport.I’m missing something here, are you saying in France people have to use air and not allowed to use helium. And there’s no narcoses at decompression depths. I’ve heard it said all the time “if you want to go deep on air” but that’s not how it works. Where you want or need to go happens to be deep and air is the gas you have to use or choose to use. If you want to dive a certain wreck or reef you don’t get to decide the depth it’s at.
You can find operators who accept other certs. PADI and SSI are accepted by, I'd guess between 1/3 and 1/2 of the operators on the med, in big cities. I don't know how they swing that legally. GUE and IANTD are big in Gramat/Lot. Other than that, it's FFESSM cert, or you're not going below 20m, if you go with a dive shop. Shore diving isn't huge here, along the med coast.
As to how FFESSM runs things, you can do nitrox as N2 but trimix is only for experienced N3 divers.
So, no, unless you dive with one of the, "niche" agencies as my local club's head called GUE, operators, you can't dive on helium unless you're N3 which means 60m on air. Same goes for rebreathers.