Played around with this for a bit, it's a well made app with a lot of charm, as expected from Nintendo. Also runs as smooth as butter on my nearly 3 year old iPhone 5s which is nice.
Though it does have the usual bugbears expected from big publisher apps on the platform - stuff like in-app updates instead of those issued by the App Store, a dependency on Nintendo/DeNA's server (it can't really be realistically used offline, instead you get kicked to a communications screen if don't have a connection when you launch the app or resume it from a task switcher on both iOS and Android.
So you can't compose draft answers or browse the app when offline - not ideal if you're travelling. But it *does* still function if you lose connection while you're in the app -- much better than most mobile games dependent on server connections.
Also, seeing Nintendo's achievement system (My Nintendo Missions) pop up as these motivators for My Nintendo points is a bit saddening. It's an admission that Nintendo has to "embrace" business science as game design as that's what the average person is engaged by. Real shame.
The game is practically region free. If your Nintendo Account is set to Japan, the game shows Japan-specific content (like the Miifoto contest), but it's all in English.
You can change the country setting on your Nintendo Account and this will reflect in-game. So game content isn't tied to region, and regional content is independent from the language of the game and the accents of your Miis -- those are likely determined by the first connection to the Miitomo servers or your phone's language settings.