Some of the French clubs are assigned to odd home grounds, mostly the tiny "Le Parc" in the '21 mod, here's my suggestions for them instead:
Bordeaux - Bordeaux, supposedly the stadium in-game is the one used for RWC 2007, which in real-life is now Bordeaux rugby's home ground, though the R08 model looks nothing like the real thing.
Brive - Le Parc?
Castres - Castres, Clermont - Clermont, both these teams have the wrong ground set in the mod, there's an odd glitch in the assignments that makes some of them "off-by-one" in the editor.
La Rochelle - Nantes? No real similar ground atm. Maybe use vanilla Vicarage Road in-place of Nantes? Would have to test this.
Lyon - Lyon, the R08 vanilla model is Lyon rugby's current ground and it's a good likeness, though they are planning on moving to a new ground in the future.
Montpellier - Marseille, It's vaguely similar to their current ground with the semi-circular stands.
Pau - ??? Some sort of "box" stadium.
Racing - Parc de Paris/Parc de Princes, mostly because I'm in the middle of making their home arena model to replace PdP.
Stade - Stade de Paris their vanilla home ground, mod currently has them at Stade de France, again "off-by-one" error with the stadium list in the editor. I might be making a model of their new ground to replace the default R08 one.
Toulon - Perpignan, Toulon replace Perpignan in the mod and this is already set for them, no need to change.
Toulouse - Toulouse, the mod comes with a replacement model for Toulouse's home ground need to check again what it's for.
There's also Lens stadium which I haven't assigned to anyone in this list.