Well it actually depends on what you want to build.I asked this question myself, I am a Javascript developer working in nodejs,react,angular etc. I answered this question myself by thinking what I want to build/learn in coming year . Finally I came with below list.
- Python : python is a great language for machine learning if you want to build some recommendation system etc. Basically all machine learning stuff from NLP to Neural networks.
- Javascript : Since I know js I can build a web app TO mobile app with javascript. React js is one the most popular languages of 2015-2016.So if you don't know js and want to build mobile/web apps go for it .From career point also it makes sense because js is on FIRE
- elixir-erlangVM : I love functional programming .And elixir is new functional lang with great syntax i guess most ruby developers will love it . I want to learn elixir because it runs on erlangVM by far the most robust ,fault tolerence,high availablity .Have a look at whatsapp stats https://blog.whatsapp.com/196/1-million-is-so-2011.
- GO : Google's open source project has become very popular recently. You can expect the community to talk more Go in 2017.