Coming from a GameSpot interview with Nintendo's Bill Trinen...

Why the original DS title never got localized.

"With the original DS version we looked very hard at potentially trying to bring it over to the US. We've had Mii characters for a while and you've been able to create and play as a Mii character within a game, but typically their actions and what they do in that game has really been limited to some physical motions, they've never really had their own personality or their own voice.

In Tomodachi Life, a very key component of the game, and what's really important to you as a player in terms of building that connection with these Mii versions of your friends and family, is that they have their own personality. Each individual character has a voice that you can customize, and they'll actually speak. That's done through a voice generation library on the software. We weren't able to do that on the DS in the US. It's much easier to do in Japanese because Japanese is a phonetic-based language; English is much more difficult.

So part of the reason that it didn't come out on DS, and part of the reason that it's taken a long time for us to release on 3DS here in North America has been all the work that's put into creating this library to handle English and the other languages and to get it to work well in a way that helps you connect to those characters and helps bring their personalities to life."

On blocking vulgar language

"There's a universal list that exists at Nintendo that will prevent you from using those types of words in a character name or something. In Tomodachi Life, there are probably fewer restrictions this time because we didn't want to block people out from using their actual names. But it's more of a development side library that they have, rather than something that the localization team actually does."

Full interview here