Wow now that's pretty surprising, looks like they didn't even bother updating to more suitable file formats for the pc version of the game. I'm guessing all of the text and stuff is in that big DAT file, using a custom file system. You'll have to reverse engineer how files are stored in that to get anything out of it.
If you have the game for the ps1 you could try replacing the dat file from the english version over the pc version to see if it can still use it