List of Japanese software
Microgaming (stand alone): Spin Palace, King Neptunes, Gaming Club Japan, 32Red and others.
Mircogaming (multi language version): Ruby Fortune & Crazy Vegas
Cryptologic: InterCasino
Random Logic: 888.com
Playtech: many casino brands
Grand Virtual (AWA): all brands
G-Fed: all brands (I have not touched them for ages though)
BossMedia and RTG do not have Japanese software.