Java language
Android is mostly written in Java. Although a few alternatives have appeared lately (we can come up with Kotlin and Android, which is a fantastic combination, for example), Java will likely remain the language of choice for Android. Its very mature environment, massive support from Google and other companies, and the vibrant developer scene, ensure it goes on leading Android's development.
One factor that did attract developers to the Android ecosystem was precisely this shared usage of an existing language. Java has some particular characteristics and techniques that we need to learn in order to use it effectively.
