Well, Google has to do extremely little to support Kotlin, as it runs on the JVM, does so fairly leanly, and does so on Java 6 JVMs.
Swift, they'd have to redo just about their entire stack, as Swift doesn't run on the JVM (Swift can be used with the NDK to build NDK libraries, but it's not standard to build a full app, outside of games, with that).
Scala has had big issues with the size of it's library that still haven't been resolved, and the new versions of Scala require Java 8, which only the beta version of Android supports right now.
Swift, they'd have to redo just about their entire stack, as Swift doesn't run on the JVM (Swift can be used with the NDK to build NDK libraries, but it's not standard to build a full app, outside of games, with that).
Scala has had big issues with the size of it's library that still haven't been resolved, and the new versions of Scala require Java 8, which only the beta version of Android supports right now.