Our clients at http://cookiedev.com
often ask about technology stack we'll use for their mobile app, and become surprised when we say that we'll use Kotlin for a native Android app (most still think that Android apps are written on Java).
In the meantime, Kotlin was announced at Google I/O 2017 (sick!) as the second official language of Android app development.
What you need to know about KOTLIN vs JAVA in a 6 minutes read article: https://codeburst.io/kotlin-vs-java-the-great-war-of-android-app-development-68bb4f1ed534