If I remember correctly the japanese IME from the Japanese Magic set is floating about.
Do a search for iWnnIME. (Note: no font replacement necessary, the Droid Fallback font has Japanese inside)
(this seems to work on my unit, don't know about you...)
Note that this does NOT fix a bug that's been around since 1.6 that affects your music player. The Android Media Scanner software doesn't handle non-English characters correctly, and hasn't done so for, um, ever.
To the other side... and beyond. <_<
Galaxy S2 i9100 Chameleon 3.0.3 JB / Dorimanx 8.35
Ouya (stock rooted)
Liquid Community ROM 1.4 (Acer Liquid)
O2 XDA Atom WM6.1 Kuruma Final
HTC Touch Elf Opal Ultimate 3.2.1
LG KS20 Kiss20.4