This hasn't been confirmed yet, however, I have looked into this mystery for quite a while since the debut of X and Y and I think I can safely confirm that the Pokemon have to be region-locked from different-language cartridges. I think it's rather evident as to why as well.
My Illogical Theory
Firstly, it says on the cartridge which region it comes from:
If you look close enough, below the 'CE' letter thingies, reads 'EUR' for a European cartridge. This appears on all Pokemon games, and the Masuda Method depends on the country that appears there, really.
This is most likely the same with X and Y - the language won't matter. It will still depend on the region lock.
Next, I think this might mean something:
If both Pokémon are foreign to the cartridge but are both from the same country, then the Masuda method will not take effect. - Source
So different-language Pokemon from in-game trades like from Lt. Surge in HGSS, are still considered as the same country, and will not contribute to the benefits of the Masuda Method.
Now, in Pokemon X and Y, different-language-same-country Pokemon appear all the time if you choose Japanese for your language on an Australian game - this is just like Lt. Surge giving you a foreign Pokemon - it will still be from the same region, just a different language - therefore nullifying the effect of the Masuda Method here.
And a less powerful point, finally it seems rather nationalist for a different-language game to benefit the Masuda Method. There are plenty of people in the world that don't speak English, but still live in an English-speaking countries. This seems a little offensive to the population of people who are in the mentioned circumstances.
It should also be noted that the Masuda Method was made to enhance the use of the GTS:
The Masuda method was likely coded as incentive for players to use the GTS's international trading services.
Anyone who has two copies of the game can make one English and one Nihongo, and they can just trade over some Pokemon and BAM! Instead Masuda Method. It seems WAY to easy.
So to conclude, if trends continue, then the Masuda Method depends on the region the cartridge was from.
Hope I helped. :)
Source: Above Link