Sorry a little confused. Casual to me is jeans and a shirt. For a foreign crowd I think you might have to be more specific (what's elegant/casual for an Italian might not be the same for an American, etc.). Maybe you could say
The dress code is casually elegant. Jeans ok.
I think casually elegant is a common expression which I've seen on invitations, especially outdoor events, which gives the idea that nothing terribly fancy but nothing too casual either is required. This is easily understood by both men and women.