The programming language widely used for Android app development is Java. Java has been the primary language for Android development since the platform's inception. Its portability, robustness, and extensive ecosystem make it a popular choice among developers. Additionally, Kotlin, a newer programming language that is fully interoperable with Java, has gained popularity due to its concise syntax and expressive features, improving developer productivity. The combination of Java and Kotlin allows for the creation of powerful and efficient Android applications.