Do you just want to be able to display and enter Japanese text, or do you want the entire user interface (menus, dialogue boxes, messages, etc.) all to be in Japanese?
If it's just text display and entry, then do this:
Start -> type "regional settings" into the search box, click "Change the date, time or Number format".
Click the "Keyboards and Languages" tab
You can then use the "Change Keyboards..." and "Install/uninstall languages" buttons to add Japanese input and output.
I haven't personally tried the "Choose a display language", but I suspect that's the way you change the display of menus, dialog boxes, etc.