what language is most important for creating applications......??

Answer 1

Android - Java / Kotlin
IOS - Swift
Windows - C# / C++
macOS - Objective-C / C++ / Swift

Answer 2

Resources for App Development

Using Android Studio

language support: Java, Kotlin, C++

https://www.sololearn.com/discuss/823823/?ref=app

Using Visual Studio with Xamarin

language support: C# , C++

https://www.sololearn.com/discuss/1068494/?ref=app