There are 3 key languages that are used for building an app:

Java has been the language of choice for mobile app creation based on Google's Android platform. Java is an extremely common programming language that enables cross-platform support and ease of portability when developing applications for different types of hardware and OSes.

Kotlin, which is the newest of the programming languages highlighted in this list, was designed to completely interface with Java.

Swift, is basically used for ios applications.