The default installation of OS X doesn't come with any other non-European language other than Japanese. Well, we all know this by now. However, there is a way to view Korean, Chineses and others by moving the respective fonts from the OS 9 fonts folder (or any other) into the fonts folder in /Library/Fonts [editor: corrected per comments] and voila!
Now go to the International setting in System Preferences in OS X and see your language displayed! Select the language and keyboard you need and now you should be able to view these foreign languages in Mail.app, Web Browsers and others.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010407155435265