Success!!
If in future, anyone ever Googles this thread, here was my big mistake. I was trying to use the action replay codes on an iso of the Japanese bonus disc. That was never going to work. If you want a Japanese Celebi, then get the bonus disc for sure, but if you're after an English language Celebi, look no further than your own US or PAL copy of the game. The action replay codes below unlock Japanese bonus disc that has been leftover in your copy of the game.
USA
Europe
5M54-WC2B-QHBG6
WNR8-RY3Q-29P6T
GMNK-UQNE-QR78H
WHZ0-EWPJ-12BTD
NRTU-8KHF-1PZZ8
DEED-TK1H-U6N5P
BHDF-N90M-BKMPV
3V7P-NZA1-ZBNVQ
WNR8-RY3Q-29P6T
GMNK-UQNE-QR78H
WHZ0-EWPJ-12BTD
NRTU-8KHF-1PZZ8
F57Q-1RNH-ZBAKF
CNU4-TJFW-TR1W0
This is actually way simpler, as no hex editing is involved, no mucking around with different formatted memory cards. Just put in the AR code and presuming you've caught and purified all 48 pokemon, when you pick the shrine option on the right, Celebi will be sent to your party in the language of your save game (in my case, English).
That said, the PAL version is insanely buggy. The cutscene is a rapid fire mess of empty text boxes. Not to mention the pokecoupons menu on the left.
Next project is the Pikachu from the pokecoupons menu. Not sure if that will be possible due to how buggy the menu is, but I'll post my results if I have any luck!