When the promo videos were shown in 2011, that's the first thing I thought would happen- the 3DS has a microphone, built-in, right? Yeah, but sadly, so does the Wii U gamepad, and Splatoon shows Nintendo doesn't do voice chat anymore, despite Wii Speak having that microphone you could set up near your tv. They even advertised it!
It obviously shows Nintendo doesn't want people screaming obscenities within their games (their claim on Splatoon's lack of voice chat), and I can't say I blame them, how would you like to tell a visitor on Club Tortimer that you're not giving them any bells, only to be cursed at? I wouldn't like it very much, and Nintendo wants their players to play in 100% safe environments, so bye-bye voice chat. Also, if you were going to have voice chat, with the insane amount of Japanese players who play Animal Crossing and Splatoon, you should probably learn how to speak Japanese!