What language should one learn in 2017 with a main purpose to create mobile apps?

Swift (iOS) or kotlin (Android)

@theZorgEffect that

Assembly

@theZorgEffect huh. And here I've been messing around with Java + XML on Android.. hmm

@theZorgEffect what is that kotlin language exactly? And how about just plain java then?

+1 for swift

Kotlin had me at semi-colons are optional.

Swift + Java

@MrDhont think of it as an much more elegant version of Java minus most of its annoyances. See for yourself: kotlinlang.org

Why choose kotlin over Java? We'll if you're not impressed by the looks, let these numbers speak:
~30% less code in general
~90% less crashes

That's what we managed to achieve by moving all of our Android & Backend Code to kotlin

Gonna learn some react native!

Why doesn't anybody prefer Cordova and node.js in it, you can deploy your app on multi platform like iOS, Android and Windows ?

Javascript & React Native definitely!