Answer 1

you can learn java, c#, c++, kotlin and many more, your selection will be determined by the kind of app you want to build

Answer 2

To programm ios Apps: Swift
For Android apps: Java
For all the rest: python ;)

Answer 3

All most all languages support

Answer 4

Java and Kotlin for Android apps.
Although, most people will prefer Kotlin to java, I think it is best to learn all of them.
C++ ,C and C# are also sometimes used especially in game development.
Swift for iOS.