Dart: https://flutter.io

Go: https://github.com/xlab/android-go and maybe https://github.com/gonativeio/gonative-android

Kotlin was an easy win for Google. They didn't really add support for it, but merely bless the support that already existed (and with that blessing comes some assurance that things won't break for Kotlin in future versions of Android).

JetBrains was already targeting Android as a big selling point for Kotlin because Android was still on Java 6 and lacked lambdas and streams and all cool new things Java has gained. Kotlin is also very close to Java. It adds some niceties for sure, but they worked hard to make Kotlin very close to Java. Data classes are great, but you could easily turn a data class into a POJO by generating the getters/setters/equals/hashCode/toString. The lambdas are great, but you could easily turn that into a Java 8 lambda or a Java 6 anonymous function. You might lose some elegance in the conversion to Java, but there isn't a lot of ambiguity in the conversion.

Kotlin support is relatively easy for Google. It already existed via the normal Java support. Supporting Go and Dart means supporting an entirely parallel set of things.

When they have a repl maybe they can be considered.