There are many programming languages available for Cross-platform app development, and each and every own strengths and weaknesses. The most popular languages for Cross-platform app development include React Native, Flutter, Xamarin, and PhoneGap. React Native, which is developed by Facebook, is a popular choice for JavaScript developers. Flutter, developed by Google, uses Dart programming language and is known for its high performance and fast development cycle. Xamarin, developed by Microsoft, uses C# programming language and is often used by .NET developers. PhoneGap, developed by Adobe, uses web technologies such as HTML, CSS, and JavaScript. Ultimately, the choice of programming language and framework for cross-platform app development depends on various factors such as the development team's skills, the app's requirements, and the project's timeline.