Simply copying the Jap content folder and renaming the files to "EUen" or whatever is likely to just give you a similar result to just using the old method, i.e .bars causing problems with the overworld voices, albeit with far more hassle.
My idea is to replace your code folder with the Jap one, then copy over the Jap Voice folder and TitleBG_JPja.pack (which also contains some voices) However, there may be some problems with the font, and if you replace anything in the code folder, the ticket becomes invalid, and you'll need sigpatches to launch, IIRC. You might be able to replace the original ticket with the Japanese one, but if launching sigpatches every time doesn't bother you shouldn't need to.
EDIT: Actually, replacing the ticket wouldn't make a difference, since it's an out of region game. You'd still need to launch a CFW. Don't bother with replacing the ticket.