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I have a leaf green. If I got a japanese verson of Fire red would it be able to trade with the U.S. verson of leaf green? Also (sorry if this was asked before which Im sure it was) is Mew in FR or LG?
I'm pretty sure you can't trade between US and Japan versions of the games. And for your second question, Mew is actually in Pokemon Emerald, via an event.
As far as I know, you can send pokémon from a Japanese FR/LG to an English version of D/P. The same way, you can trade between a Spanish and an English version of FR/LG. So the chances of you being able to trade from the English to the Japanese version are really high. Try, if you can, nothing bad will happen to your game anyway.