[pjmlp]

I would like to say Java, but given the way Google stagnates Android Java on purpose, and pushes Kotlin no matter what, Kotlin is the way to go for serious app development.

Then if you want to do anything related to 3D, realtime audio or machine learning, you also need either C or C++, as those APIs are NDK only (OpenGL is actually also available to Java, but Vulkan is the official 3D API since Android 10).

Now if you want to target Android only as hobby, use whatever you feel like.