You can play on a japanese server if you desire. The region split is mostly so people can have a server located closer to them for less latency and also to gather people that are roughly in the same time zone and a higher chance that you will speak the same language as most people on the server.

A tip if you do decide to play on a japanese server: make sure you enable the J in the duty finder queue menu so you will be paired up with Japanese players. Otherwise your duty finder queue will not place you in those parties when they are a majorty of what will be available on those servers.