I read that you can't use Japanese on AIM. To do so, you'll have to download the Japanese version of AIM, which supposedly can't be installed on a non-Japanese Windows OS. AIM isn't really popular in Japan (hence, America Online), so I think they didn't really work much on getting such things to work. You'd be better off using MSN Messenger or Yahoo! Messenger. Those are used worldwide and the language supports is large also.