Can someone tell me exactly how the Masuda Method works?

>If you breed two pokemon from different regions, the chances of hatching a shiny are increased

What I know:

If you have Pokemon A (caught on a Pokemon X/Y cartridge purchased in America set to the English language), and have them breed with Pokemon B (caught on a Pokemon X/Y cartridge purchased in Japan set to the Japanese language), they will have an increased chance of hatching a shiny pokemon

If you have Pokemon A (caught on a Pokemon X/Y cartridge purchased in America set to the English language), and have them breed with Pokemon B (caught on a Pokemon X/Y cartridge purchased in America set to the Japanese language), they will NOT have an increased chance of hatching a shiny pokemon.

But do they have to have different language tags to work? (The JAP, ENG, GER, ITA, SPA, KOR ,FRA tags on the summary page)

Will a ENG (or in my case, non-tagged) Pokemon bred on my American cartridge work with an ENG Pokemon purchased on a UK cartridge?

Will a ENG Pokemon bred on my American cartridge work with an ENG Pokemon purchased on a Canadian cartridge?

Will a ENG Pokemon bred on my American cartridge work with an ENG Pokemon purchased on a Mexican cartridge?


What exactly counts as a region?

And is there any in-game way to know if a Pokemon is eligible for the MM, besides their language tag (which apparently isn't what matters for MM)?