Python or Java as Backend Language in Google App engine?
You can really go with either, to be honest, and use whatever suits your style.
When I started using App Engine, I was Java all the way. I recently switched to Python and love it too! 
If you have a lot of existing java dependencies, such as libraries etc. that you want to continue using, then stick with it. Otherwise, it's worth dipping your toe in the Python waters.
I am developing an application in Android using Google App engine and Google Compute Engine as backend . 
I have followed the Google's demo code in python as base for my application. 
Now I have question in my mind that since I am more familiar with Java then Python and also need to consider the fact that Google is supporting Python more then Java in its most of the demo codes, Should I change my GAE backend language to Java?? 
I should stick with Python and hope that I would come around with Python eventually. 
Any suggestions are appreciated. Thanks