Gjiten doesn't need fonts or CJK to function, but as any other app you need fonts.
If it doesn't display characters you can get the fonts them by adding cjk catalog and installing just the fonts.
You can copy and paste japanese into gjiten, so you can use anything that can c&p, e.g. websites. There are also web based IM. If you want to type in directly you need CJK. It cannot use original on screen keyboarb therefore:
For n800 you need to install matchbox-keyboard!
N810 has an hw keyboard, so it doesn't install it by default.