Sorry in advance if this is redundant but I wasn't able to find anything that specifically addressed this in detail.
I did the Old Sea Map Mew on my Japanese emerald cartridge. I did the runaway method to shiny hunt it. After catching the Mew, I traded to my English emerald cartridge so I can trade up to my gen 4/5 games then to Bank. But upon doing so I realized that my Mew doesn't retain the Japanese language tag at all and my gen 4 and gen 5 games see it as an English Mew.
My question is, how exactly do I maintain that Japanese tag to trade it all the way into Bank and then Home? I've come to the conclusion that I definitely need a copy of Japanese heartgold/soulsilver or Japanese D/P/PL. But do I then also need a copy of Japanese pokemon B2/W2?
Let's say I migrate old sea map Mew from my Japanese emerald cartridge to a Japanese HG/SS cart, then trade that Japanese Mew to my English HG/SS cartridge, will the Mew then retain the Japanese language tag at that point as i trade it up to Pokemon Bank?
Or do I need to further migrate from Japanese HG/SS to a Japanese B2/W2 cartridge for it to retain the info?
Thank you in advance!