Hi,
To dive in France you must be certified, whatever card you have.
To dive a rebreather, you must be certified on the gas you are breathing. And it must be CE approved.
This is when you dive in a dive center. When you are on your own, you are free to do what you want.
Most technical agencies (TDI, IANTD, etc...) are recognised. Anyway, if you want to dive with your rebreather, there some smart people do accept non CE stuff.
An instructor in France can deliver a temporary certification (only valid in the dive center) based on the level and the skill of the foreigne diver. So everyone is happy!
