You're totally right. The trailers even shown some scenes that weren't in the original, so you can bet there's plenty of new text in there.
Either way it wouldn't require a simple 'copy/replace' procedure, depending on how the game is coded replacing the text bank also sometimes mean that the text has to fit in the same amount of letters/characters. Menus and items are usually easy to translate as they aren't fetched from a 'text bank', but rather hard coded to appear as such. As you can see in the screenshot, 'magic' is reduced to 'mage' probably for that reason.
Already having the items/spells and such translated would probably help people get through the game from memory, to enjoy the japanese voices, until we get an official US release.
I'm suspecting an announcement at E3 this year.