At a guess, Metropolitan French. Lumiose is modelled after Paris, the capital city, so even those from rougher neighbourhoods whose language is rougher and less formal would probably, in a real-world setting, not be using any more obscure regional dialects. I assume this is where you were going, because in-game they speak whatever language you're playing in. So I'm fairly sure the answer is the same as what is standard for most of mainland France irl, which would be Metropolitan French.
Source: Wikipedia
