Like irmongoose said OS X natively supports any major languages without installing any additional software. I use Japanese and English all the time and I've seen Chinese and Korean characters properly displayed.
But here's the catch:
Many apps developed in Roman language countries has fixed English fonts assigned to display characters. If this is the case with your software, there's not much you can do to display non-Roman characters. If your app uses system fonts to diplay characters you usually have no problem displaying non-English fonts.
If that software is OS 9 capable, you can use shareware like Font Patchin' to force any application to use user specified fonts. I don't think there are any software that can replace any application fonts on OS X yet.
When you are on Classic environment (OS 9) you need to have a specific language OS and/or Apple's Language kits.
Too many ignorant software developers don't realize that there are many guys wanting to use 2-byte characters like Japanese within English-menu-ed apps. They should stop using fixed application fonts unless it's absolutely necessary and make them capable of handling 2-byte characters, expecially on a multi-lingual OS like OS X. They don't need to localize the whole app and manuals. What we need is an English software with 2-byte handling ability! FastTrack Schedule has recently updated theirs to support non-Roman characters, which I'm very happy about. The next one up on my wish list is SplashData's SplashWallet suite. Their Palm apps work fine, but the Mac OS X apps eats up synced Japanese characters data! Oh and the server software PHP 4 supports native multi-language functions. Yes!!!
Multi-language apps rules!
Here's a very very frustrated guy in Tokyo...
I've been in pain since my OS 6.0.2 days about this.