We do use it, but the saying is, "It's raining cats and dogs", not "like cats and dogs". We also say "it's pissing it down" (vulgar) - "it's chucking it down" - "it's torrential" Why we don't just say it's raining, I don't know haha There is a myth, that during history there have been storms that actually rained down frogs and fish, if it's true or not I don't know but the cats and dogs, and frogs myth could be connected. That's an excellent answer hahaha

En Français, on dira "il pleut des cordes"/" il pleut à verse"/" il pleut des trombes d'eau"/ etc... s'il pleut vraiment très fort. :D

Australians say this as well when the rain is really heavy.

Danish: det regner skomagerdrenge ("shoemakers' apprentices") hahahahahaha

http://en.wikipedia.org/wiki/Raining_animals