Depends on what you mean by "don't understand". If you mean any language I'm not reasonably competent in, then it would be a toss up between Chinese (pragmatic motivator, although the script is beautiful), or Arabic or Persian (revelatory languages of the Baha'i Faith, whose script is also beatutiful).
But if by "don't understand", you mean that I've never even tried to learn them up util now and so possess no more than a few words if that, then maybe Italian (it just sounds good. But I must say that really, all the languages mentionned above sound beautiful. Then again, I think all languages sound beautiful (except maybe English just because I hear it all the time, so it doesn't sound so interesting anymore, sinse I've just gotten bored with the sound of it. Had I not been exposed to English all my life, maybe I'd find it beautiful sounding too.
