Many Western games don’t see Japanese releases namely because they aren’t as popular among Japanese gamers as the local games.
Assassin’s Creed III is one of the few games that’s getting a Japanese release, and Ubisoft has released a video accompanying the announcement. They’ve also launched an official Japanese website for the game.
It’s not very likely that the game will see any sort of localization apart from subtitles, however—most of the dialogue is expected to remain in English. By comparison, European versions of Assassin’s Creed III will be getting full voice localization.
You can watch the trailer of the Japanese announcement above the fold.