Some interesting stuff. Importing the bdat files from the English version into the Japanese one works, and you'll get English text, but the formatting will be all over the place. The menus and everything else will still be in Japanese, most likely because they're in the common_ms.bdat (and common_local_us.bdat?) or something, which for some reason didn't work when I tried to replace it in the patch0.cpk. I have no clue why that's the case, but it might depend on some other file from the English version which doesn't exist in the Japanese one, so anyone interested can try figure that out I guess. The text formatting might also be stored somewhere else, and a good place to start might be the scripts folder, since there's a bunch of files named the same as the .bdat files. Right now these two issues basically makes the game not playable. Font can be replaced with English fonts, but this doesn't solve any formatting issues, unfortunately. So yeah, if this works we'd basically have the Japanese game in English, with correct lip syncing and all that.
Another interesting thing is that it seems you can create a patch0.cpk for the English version, and replace files through this, similar to how the Japanese version does it with the patch. The way I tested this was to create the folders "\menu\bmn\us\" and placing "system_avatarmake_00.bmn" inside, before creating a patch0.cpk. This should replace the background during the avatar creation voice screen:
http://i.imgur.com/auy3XpT.jpg The selection itself doesn't work though, as .bmn is just the background image used there. I guess the menu edits are in the rpx, and I did try to decompile it to an elf and looking at it in a hex editor, but well, I had no clue what I was looking for or at so I gave up
Aaaand it's super late (or early depending on how you look at it

) here so I'm going to bed.