It's not that you need one from your game, but it's a lot easier to have one foreign and one from your game, so long as they're from different regions. In your example, the Spanish Pokémon and the American Pokémon would still trigger the Masuada Method.
> The key to using the Masuda method is not to breed any two Pokémon, but to breed two Pokémon created in games of different countries. An Egg resulting from such a pairing will have a higher likelihood of being Shiny. The most common way to arrange such a pairing is to use one foreign Pokémon and one from the game in which the breeding occurs, although the method will work in any game provided at least one of the Pokémon in the pair is from a country different to the country of the game cartridge.
Hope this helps!
Source: http://bulbapedia.bulbagarden.net/wiki/Masuda_method
