actually, i played my PSP2 US save data on PSPi, you just need to rename the US save data folder as a JP, then import the save data in-game, i did this on PPSSPP though, dunno about real PSP.
Are you sure...? I have seen tutorials about it involving the use a plugin to decrypt the save data files and other things that now I don't remember... Also, I've seen maaaaany threads (in several places) with ppl asking for such a service, and offering. (I think here in this thread too.)
It seems more complicated than just rename. Could someone confirm this?