When the cat is not around, the mice dance. The problem is: How do those poor mice know that the cat is not around? Unless he was seen being hustled away to the vet in a carrier, the typical mouse would be hard pressed to know for sure.
Anyways, there's the same idiom in English but it reads a bit differently: When the cat's away, the mouse will play.
It's not a mouse and it has little to do with idioms (actually, it does - see later) but the French name for Tweety, the yellow canary, is Titi. What about Sylvester, the cat? It's "Grosminet" ("gros" is big/fat and "minet" is cat/kitty).
That's "Titi et Grosminet" (in French). Titi always says "j'ai cru voir un rominet" (yes, that's "I thought I saw a pussycat") whenever Grosminet is near. I guess that qualifies as another idiom.