You know, I've installed Japanese rendering support before and whenever I've messed with reconfiguring locales, something always went horribly wrong.
If all you want to do is be able to read Japanese text from webpages, etc. then you just need to install the right font packages (I think that link lays it out pretty well) and maybe modify your /etc/X11/XF86-Config4 file to change the FontPaths. You only need to mess with locales if you want the menus, dialogues, etc of your applications to render in Japanese (which I don't). There are a lot of webpages out there (some which are grossly outdated) and they can conflict with what you tell you, so sometimes it becomes very difficult to know what to do.
Japanese input support isn't too bad though. I typically install uim (universal input method) along with anthy, but you could also install scim (simple common input method). scim basically sits above uim, and any other "lower-level" input editors and manages them all. I believe there's a little bit of a performance overhead with using scim though, and since Japanese and English are the only two languages I know/care to write in, uim works fine for me.
