Hi Kurisu and all!
You have absolutely no problem all with that, as I don't.
I got an iMac DV+ here in the U.S. so it came with English version of Mac OS. Was it since 8.5 (or 8?) we no longer had to buy "Language Kit" to deal with application in foreign language, such as Japanese version. I felt a bit ripped-off since I already bought a copy for my Performa 6400/180 separately almost 4 years ago, but I liked the fact that kind of gave me the sense Apple cares about the rest of the world. (FYI, as long as Language Kit is in, you can run applications in your language which you can't with Micro$oft, well, a few exception with Win2k+Office2k) Now till 9.1, all you need to do, is go through "custom installation."
Ok, enough intro ... Only difference would be the languages on package and those physical documentation, such as manuals, depending on the region you buy your copy in. During the installation, it gives you the option in which language you prefer, then pick "Japanese." Then the rest of the process will be explained in Japanese ... btw, default is English so I guess I could have done that way too.
But it didn't tern my English Mac OS 9 into Japanese version, so it actually make sense to see the English interface on Mac OS X first (this must be the typical behavior of "One Application Package" Steve Jobs claimed ... the matter of fact, open IE, then you'll notice it also becomes Japanese version). But conversion is like a charm: "System Preferences" --> "International" --> "Languages" then put Japanese on the top list if you like Japanese on menus, dialogs and stuff. After you reboot, Voila!! Now you got Japanese version of Mac OS X (and I didn't need to have it shipped oversea)! This is also a way to switch other 6 languages (English, French, German, Italian, Spanish, and Swedish) back and forth. It's easy to imagine to have full support for Roman-based languages (1 byte characters), but I have my hats off to Apple ... for giving away Japanese support (2 byte characters). Bravo. Can M$ be as generous as that!?
Pardon me I couldn't keep it short ... got a little carried away to see another person dealing with Japanese (at last)!
cyberkazoo
(an International Student from Japan)