Blu-ray will have room for both the Japanese voice track and the English one. Wonder if you select Japanese audio if you have the option for subtitles. If so that’s how I would play the game.
In a recent Q&A with IGN, FFXIII producer Yoshinori Kitase explained that without the kind of memory available on Blu-ray, [...]
"PS3 Version of Final Fantasy XIII will have 2 audio tracks, 360 only 1" - 13.05.09