For me, it's python and node.js (backend javascript). I feel like those are two languages that are everywhere these days. I'd also love to get into websockets or server-sent events to replace the long polling PHP chat we currently have going on (eek).
A couple of years ago, I tried playing with SSE (text/event-stream MIME type) and for some reason I couldn't get it to work despite following every tutorial out there. I think that it must be something about the way our nginx servers are configured perhaps?