I routinely work on iOS (Swift, ObjC, some C), Android (Java, some C/C++), Xamarin (C#), Java Server, and .NET server. Lately we've been using Swift 3 server for some POCs. On the mobile side, you have very limited choices.
You can do native, which means Java on Android and Objective-C and Swift on iOS.
You can do compile-to-native, which for me is C# on Xamarin, compiled to the native languages (C on iOS and C#Mono to JNI to Dalvik on Android)
On the server side, your choice is generally dictated by whatever the architect who designed it decided in the beginning (if you are lucky, this was you). Everyone comes in and wants to rewrite the ancient thing that sits on the backend, but it's often not worth it. I love J2EE but it's not what I'd recommend today for a new project. We're actively exploring Swift on the server, but it's not really production ready yet and won't be until the fall at least.