However, for the intrinsic joy from developing something on an Android device, I'm particularly interested in the language Mirah created by Charles Oliver Nutter (the developer of JRuby). I keep Mirah in mind, for whenever it will become practicable to use in Android development.
Perhaps Mirah is ready now—a Google search shows plenty of people using Mirah on Android.
Back in 2011, Nutter wrote an article about Mirah in Dr. Dobb's Journal. To sum up, this is a Ruby-like language designed in such a way that, for any program feature (or user feature) a programmer desires to implement, she would create a new keyword or plugin for the language (somewhat directly in the compiler—this being Mirah's main point of departure) rather than add a new Java package. And, it compiles to the JVM.
Therefore in Mirah (unlike in JRuby-based approaches such as Ruboto) no program needs a package or library beyond what is already in standard Java and Android. Programs written in it are extremely tiny (due to running without huge, additional language libraries) and they load and run (both) extremely quickly on Android devices (with their Dalvik Java virtual machine).
To use Mirah to build an Android app, see the Pindah project.
In order for someone to use Mirah (essentially syntax sugar on top of Java) BTW not only should they be familiar with Ruby, but they must also know Java well.
There's more about Mirah here:
For Android development, other JVM languages, Scala, Clojure, etc., as well as Ruboto, are also interesting.
Various StackOverflow questions relate to Android development using Mirah, JRuby, Ruboto, etc.:
Copyright (c) 2013 Mark D. Blackwell.