If most of the projects you work on use Java or Python, I would start with those. In additional to a server side language you could also focus on a server side

If most of the projects you work on use Java or Python, I would start with those. In additional to a server side language you could also focus on a server side framework that will help you in ramping up. Like http://www.dropwizard.io http://www.dropwizard.io for example.