Literally, it's "Sa langue prend de l'avance face à son cerveau" or something like that but the equivalent expression in French is "tourner sa langue sept fois dans sa bouche" which means you have to think about what you will say before to say something bad.

In french there is an expression often used at the past tense when you want to apologize for having said something a little nasty that you didn't really think (when you're angry, for exemple): "Mes mots ont dépassé ma pensée" or "ma parole a dépassé ma pensée"