When Gamereactor first reported that Monster Hunter 3 Ultimate would be region-locked it caused a bit of a stir with the community. Now Capcom and Nintendo have listened and the issue will be sorted with a patch in April.
"Capcom and Nintendo have today confirmed that following the 22 March release of Monster Hunter 3 Ultimate for Wii U, a software update will be released in April as a response to Capcom community feedback.
The software update will add online cross-region functionality, allowing European and North American gamers to play together online in up to four-player battles*. Monster Hunters will also be able to play the game using just the Wii U GamePad without the need for a TV screen as the software update makes the title compatible with off-TV play."
Due to the how different the Western versions of the game are compared to the Japanese edition we won't be able to play online with Japanese players.