For Europe, they have to translate into five languages, convert to the different TV format(they're not just going to rip off all the old TV using people because your new TV supports US formats or whatever, I assume), do five different languages worth of voice acting for the Pokemon and such, test all five languages so we don't get another Mario Party spastic event, then delay it a month after it's actually fully done so there's enough copies for everyone to get it. They also have to make sure the 20 some years worth of game character names and game names apply to the region.
Right?
Mario Kart just requires some menus/descriptions for menus and missions, PAL adjusting, and online compatability. Wii Fit requires the same, but rather then online, it would require a lot of voice acting... but it would also probably be easier to test then Brawl, doesn't have 20 some years of game history to adjust, and serves as something to make up for other things(like Brawl) I would guess.