翻訳 - フランス語-英語 - méfie-toi de celui dont le ventre ne bouge pas...

I believe you meant "laughs"?
Or even better, "... when laughing."

obviously, you're right. i'm sorry for the mistake, i meant "laughs".

beware of the person whose belly doesn't move when he laughs

I always try to stick as close to the text as possible, and only change it when either:
- there is no exact equivalent in the target language for the original expressions in the source text, or
- you can see that there is a perfect expression in the target language to express the meaning of the source text, but the source language has no expression for it. (However, this second one is sometimes dangerous, because you're assuming you know exactly what the translator wants to say)...

thanks a lot for your suggestion ian, i'm going to check the translation