If you want to land a job as fast as you can, you should get a team-enabling language like Java.
If you have some project you want to get done but need backend knowledge for it, go with a mature high productive toolset. Choose either Python+Django or Ruby+Rails and grow from there.
If you want a new area where you can have fun and learn new stuff, go wild with non-mature high learning curve tools like Closure or something even less mainstream.
If you have some very specific requirements like high availability, high concurrency, or whatever, ignore everything above and choose your toolset based on those.
> you should get a team-enabling language like Java
Note that if you want to work for a startup, Python, Ruby, or JavaScript are much more popular languages today than Java. You can iterate much faster in Python vs Java.
I want to change my current development role and put myself into different position to the end of the year. I want new challenges and I want to try to work in different environment – from sandboxed browser to real servers (or containers), with different security model in my head, to work with databases, and I'd like to work with distributed system as well.
I think you should take a VPS, one of (Django || Rails), do a small project all by yourself, and decide if you want to go deeper on any of those things. The alternative is to learn Java and go quickly into an entry-level job, but you won't be able to choose what to learn by doing that, besides the small project will help you learning Java too (but may make it unbearable).
If you want to land a job as fast as you can, you should get a team-enabling language like Java.
If you have some project you want to get done but need backend knowledge for it, go with a mature high productive toolset. Choose either Python+Django or Ruby+Rails and grow from there.
If you want a new area where you can have fun and learn new stuff, go wild with non-mature high learning curve tools like Closure or something even less mainstream.
If you have some very specific requirements like high availability, high concurrency, or whatever, ignore everything above and choose your toolset based on those.