How do you say Who cares In french translation?

"Who cares?" may be translated as "On s'en fiche." (we don't care) or "On n'en a rien à foutre" (same, in stronger slang.)

There is no direct translation for the interrogative form that I can think about.

Sometimes people ask "À qui importe ça?" but this is a proper question (i.e Which person would care about this?) as opposed to the sarcastic and rhetorical (i.e. Why should anyone care?).