"Mike Inglehart of Next Level, the company developing the title for Nintendo, explained in an interview with the UK’s Official Nintendo Magazine that due to their desire to get a “quality gameplay experience”, online matches will be region-specific. “The game is very twitch based and reactive,” Inglehart explained, “so it’s important for us to keep lag to a minimum.”

It's also explained that this limitation would only come into effect when playing matches against random players, you should still be able to play against friends from different regions, its all abit fuzzy at the moment but we'll know in a couple of weeks when the game is finally released.

[source wiinintendo.net]