Which programming language is mostly used in developing apps

Question

how to program

Answer 1

Swift is used for iOS and Java for Android

Answer 2

Apps as in applications? Any language can be used to program applications. If you're talking about mobile applications, then I suggest going with C/C++. You can utilize that within iOS, Android and Windows phones. If you want to be more specific, go with Java if you're in to Android, and Swift if in to iOS. C# for the windows phones, naturally.