If you all you need is display Japanese characters, I think I have good news for you. Just uninstall SCIM in Application Manager. That shouldn't affect the display of Japanese characters, only the input method will be gone and your thumb keyboard will come back.

When I first got my N800 I had the exact same problem and what I found out is that all you need for display of CJK characters is the font. It gets installed by SCIM, but not unstalled when you remove SCIM. More specifically, all you need is this file:

/usr/share/fonts/truetype/fireflysung.ttf

Also, if you ever need to reflash, make sure you keep a copy of it somewhere. Then put it back after installation. Worked for me!