Who said (in French) 'Let them eat cake'?

Marie Antoinette?

I´m probably wrong, that seems too easy.

You know Julieta that Fred has to be tricking us into saying Marie Antoinette!

So this is a famous mis-quotation, and the person who said it is in fact Jean-Jacques Rousseau and there is speculation that he could have been talking about Maria Theresa of Spain...see this link.

Yes, and the original words were a reference to brioche, not cake as we would think of it: "Qu'ils mangent de la brioche"