You need to change the BIOS of the system. You can either get a JAP BIOS, or get the DEBUG bios which allows you to do so much more than just change language. Drop me a line at jmkurtz@macola.com if you have any questions.
that's the MVS bios, tho.. so it would behave like a japanese MVS, not japanese home system. problem is, you can't insert coins and get into the test menu that way, I believe.