Regardless of Kotlin (which I don't feel inclined to learn at this point), would you say Scala is usable for Android development? I'm about to start a personal project and would love to use Scala if possible. Note: by "usable" I mean really usable, not "it works, but with lots of caveats and extreme pain" :)
In my experience it is.
The sbt-android plugin is really amazing. It makes compiling with ProGuard enabled as fast as Gradle without ProGuard, due to the plugins integration of incremental compilation with ProGuard.
Plus, there are quite a few built-in features and "add-on" plugins which enhance the experience further (sbt-android-protify, sbt-android-gradle).
A bunch of people built an "official" website around Scala-on-Android recently, scala-android.org. If you have any questions drop by in the Gitter channel: https://gitter.im/scala-android/sbt-android
JetBrains, as Kotlin steward, takes an effort to have Kotlin work painlessly within Android Studio.
The Scala support for Android apparently only works properly within InteliJ, which means a few versions behind what Android Studio actually supports. It also requires much more configuration steps.