I have had bad experiences with cheat codes as they had repeatedly messed up my Battle Frontier records. So yesterday I tried opening my English Emerald save file in my Japanese rom, found out that it worked fine, saved the game, dropped the save into the Japanese Tickets tool to get the Old Sea Map & activate the Mew event, opened the game back, got the ticket from the man in the communication center, saved the game, copied the save file back into the English Emerald folder, opened the game & the event was indeed activated. I did this with the Deoxys & Ho-Oh/Lugia events; both worked flawlessly. No abnormalities with my gameplay so far.