Android as a complete OS solution involves a lot of moving parts. Very broadly speaking, these parts are the app ecosystem and then the OS itself. As a developer, your programming language of choice varies depending on what part of Android you are working on.
For app developers, Java and Kotlin are popular options. For developers working on the OS and the lower levels within it, C and C++ have been popular choices so far. Today, Google is adding a third option for OS developers, as the Android Open Source Project now supports the Rust programming language for developing the OS itself.