Sega is a U.S. company that moved to Japan. Also, Sega consoles were not doing well until after the Genesis took off, which was American only. I think both Japanese and U.S. sets are necessary, and of course the PAL games that were not released in Japan or the U.S.
I am working on this one too but very slowly.
There were nearly 1500 titles released in the U.S. for SMS through Dreamcast not counting Game Gear. How many were released in Japan?