How do you say el culo in french? Derriere?
They have various terms for it, depending of the context.
Derrière is correct but mainly used as an euphemism, when one wants to avoid crude words - so rarely when talking about sex. It'll end on your ass. => Ça va finir sur ton derrière. (talking about some fat dish)
In a sexual context, they mainly say
cul. This girl has a nice ass. => Cette fille a un beau cul.
A slang word, used in very informal situations and when you do not care to look the gentleman, is
boule. Check that ass! => Mate ce boule !
Edit: some spelling mistakes.