The programming language predominantly used in Android app development is Java, although Kotlin has also gained significant popularity. Java has been the foundational language for Android programming, offering robust libraries and a strong community. On the other hand, Kotlin, which is fully interoperable with Java, provides modern programming features and enhanced developer productivity. The choice between these languages often depends on factors such as project requirements, existing codebases, and developer preferences, with both languages being officially supported by Google for Android development.