There are two official programming languages for native mobile android development:
Kotlin was declared official android language some 10–15 days ago during Google I/O 2017.
Java is official android language since android came into existence.
Java's support on internet, stack overflow, books, tutorials is top notch. No programming language in world comes near Java in terms of support from programming community. However there are programmer who used Kotlin for android development thats why Google made it official language (or maybe because of its dispute with Oracle for copyright infringement lol).
If you're just starting learning android go with Java.
If you have some experience with java the I'd suggest to go with Java and use some kotlin code too. because it can be used with Java and can use all Java libraries built for Android.
I am here not talking about PhoneGap/Cardova(HTML5,CSS,JavaScrip), Xamarin (C#), Corona(Lua) etc because I think we don't get that feeling with cross platform languages like we get from official languages.