It appears that Miitomo will begin to be available soon in the app store on your device. Official Nintendo of Japan twitter put out a message stating the launch date for its territory will be March 17. With English and six other languages to be supported initially, it would seem to point out that other language versions should not be far behind.
My personal guess is that we would get it a week later, but we will provide an update when the launch is confirmed for North America. Miitomo is the first application to be published by Nintendo on smart devices. This is a social application to find out more about your friends while acquiring crazy costumes and taking photos. This is linked to your My Nintendo Account which you can earn points to get rewards and discounts.