There are two tracks in the music library that can only be unlocked by doing certain things in Japanese.

The first is titled "Song of the Nabateans" which is unlocked by viewing the event "Rhea and Sothis" in Japanese. This event happens in game after the ball, but can be accessed from the Event Gallery.

The second is titled "Edge of Dawn" which can only be unlocked after viewing the credits in Japanese. There is no way to view the credits other than finishing the final chapter, so to unlock this one you can load a save from before the final chapter in Japanese and play through until the credits.