I imagine the translation process is a lot more work than the actual hack needed to put the new translation in. Is it against the law to translate a work of fiction for personal use?
You could do that, but you can't just apply the patch to an UMD like that. You would first have to extract the UMD to your computer, then run a patch program that would alter the contents, then move the filessback to the PSP. Now that it has been altered, the PSP will no longer run it, as its digital cryptological signature is detected as false because the files have been altered. This means you can only run the game with a PSP that accepts unsigned code, also known as a hacked PSP with a custom firmware.
It would be at least 50 times faster to just download an ISO of the game and run the patch directly on that, then move it over to a CFW PSP, because CFW PSPs run pirated games anyway, that's why probably no one would actually pay for the game to play it in english.
Or as aulayna said, just use an emulator.