There is no such thing as best back end language it depends on the area of programming you want to specialized on.
I hope this helps you to determine the the back end language to focus on!
Some software engineers did not have to choose a language. They had to adapt to the language used by their company. It depends on your personal preferences and on what you are trying to accomplish. Once you know one programming language, it is easier to learn another. Perfect practice makes perfect.
kivy (Python) for Android is used as back end developer.
Actually Python has very big support for online Programming.
Kivy: Cross-platform Python Framework for NUI Development
Open source Python framework for rapid development of applications that make use of innovative user interfaces, such as multi-touch apps.