Someone at Netflix did a talk perhaps it was about Falcor. They use React embedded in browser apps across all devices. So they use Javascript on the front end. They use Node.js as the proxy server / router with a framework called Falcor which is written in Javascript. Falcor queries all the multitudes of services and databases then caches that data in a graph. Falcor reaches out to all the servers and databases, scores or services, that are written in Python and Java.
Picking a nit, they don't use React in the browser across all devices. They actually use an internal rendering engine called Gibbon on some devices with client code written JavaScript with React but not rendering to HTML or a browser instead using their internal engine. Interestingly this has become easier with React breaking out React and ReactDOM.
Interesting to note that Netflix must see Gibbon as a competitive advantage, I've been told its not likely to see an open source release.