For the additional experience when using a foreign Pokemon, does it have to be on a different region cart (i.e.: A Pokemon bred on a US cart gains more EXP on a JP cart)? Or does it just need to be different languages (i.e.: A Pokemon bred on an English-enabled cart gets more EXP on a Japanese-enabled cart)?
I'm pretty sure it's the latter, as that's how the Masuda Method is believed to work (different languages (remember you can select your language at the start of the game), not different region carts).
It's just language.