You can start translating the module Game to Japanese language.
To do this just translate the mod/game/lang/en_utf8/game.php to your language and post it to this forum.
For hangman to work just translate the line of the above file
$string[ 'lettersall1'] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ ";
to Japanese and put to directory /mod/game/lang/ja_utf8