That depends. I use an EZ Flash IV card to trade my Japanese emulator games with my retail Emerald cartridge. There are other methods, I believe, but I've never used those.Your point is quite valid. On the same matter, how difficult would it be to transfer the Pokemon from the Japanese emulator to an English GBA cartridge (I have FR, LG, and Emerald)?
Though, if you want to RNG legends, the only thing that I think is actually useful, is Mew. You can get the others in non-Japanse Emerald just as well. Unless you want your legends to be Japanse of course lol.