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A little background:
Earth Defense Force 2 was released in English as "Global Defence Force" on the PS2. It was later ported to PSP, but the PSP version was never released in English. The Japanese name is Chikyuu Boueigun 2 Portable.

Anyways I took a look in PSP version and surprisingly there's a folder called "USA" and inside that folder is a EDF2.DAT archive. I believe it's compressed and completely beyond my skills to explore. The archive has a VFS header. It's possible that the game is completely translated and all that is needed is to modify the game to load the English archive instead of the Japanese archive.

Tried a simple copy and replace over the Japanese DAT, but of course that didn't work. Anyways just throwing out some basic information out there in case anyone comes across this post and wants to explore it further.

Sorry to resurrect an old topic, but I got this game last year and one of the first pieces of info I found was this post. After some more digging, I found that some Japanese gamers had cracked the extra archive. In a nutshell, it has the English version of the second game, plus the first game's missions (In Japanese) and a load of extra missions (also in Japanese).

I put up a guide saying how to do all this, which I hope doesn't have too much waffle.