My girl friend has a japanese computer and I want to upgrade her to Windows 7, but if I bought windows 7 in America, can I install it to display everything in Japanese? Im also somewhat worried about the drivers since I wont be able to help her as much if everything were to be in Japanese. However, maybe I should image her drive in case I want to just cancel the project and restore it.
Definitely image first!
It's likely you can find drivers for all of the hardware in her machine -- drivers tend to be pretty language-agnostic.
The only oddities might be some exotic Japanese-only hardware like a Felica reader found in some VAIOs. 1seg TV tuners might not work either -- since there's no 1seg TV here. I don't think.
I don't know if you can do an upgrade from Japanese Windows Vista to US English Windows 7, but I do know you can do a clean install of Windows 7 and then install a Japanese language pack. Ultimate and Enterprise support installing language packs to support more languages.
As far as I can tell, once a Japanese language pack is installed on a US English OS -- it is pretty much identical to running a straight-up Japanese OS.
I would install the language pack first from Windows Updates, so any driver package (or software) you install (either International or Japanese version) will default to Japanese once you choose Japanese as the display language.
The process isn't 100% reversible (once a language pack is installed, I don't think you can remove it) -- but you can flip between languages whenever you like in case the kanji gets horrifyingly complex. You can have languages per-user too, I think. So her user account could be in Japanese, but an admin account for you could be in US English.
Also, remember to choose the right keyboard mapping -- so the keys perform as they're labeled ;)