A bit of an echo from the previous posts: ds games are not region locked, but there's a catch.
If you are going to play multi-player, you might not be able to play with games from a different region. A good example is bleach: dark souls. A game was hosted by an American cartridge, but European games could not join, because they cannot see the American game.
The opposite may be true as well, the most extreme example is advance wars days of ruin. Even though all the names of the characters and units are different, you can still mix up European and American games in multi-player.
So in short, it is not region locked as a game in a whole, but it might give problems if you mix up the regions during multi-player.
(note that I have not tried mixing up Japanese games with European ones and American ones, so i can't share any experiences about those)