The concept art I linked has the Japanese name for Gekkouga in it, dude. And I guarantee you that in the eighteen months since X and Y started development and conceptualization phases, they knew what they were calling him across every language they intended to release it in, further evidenced by the fact that the game launched worldwide on the same day, without the usual localization period some games saw.. which unfortunately we get to see for Smash 'cause they can't localize the game next to the Japanese version, but I digress.
Anyway, I guess it doesn't matter since we have no proof either way, so hey. Certainly haven't seen any other prototype concept art among all the other concept art I've seen, so who knows?
I asked the same question, but if you must know, it makes me feel froggy.