But if you should sell on sites like that is another question
It's a bit biased to the banking sector; but it gives you a general idea of the skills demand (and rates!).
edit: Not just banking sector; but IT service providers. Europe (and the UK) are mostly service providers, I mean: usually don't develop IT products that they sell to others. We usually integrate and maintain stuff developed elsewhere (mostly by US companies).
I wrote a few Selenium scripts that would gather jobs from ~20 jobboards/freelancer sites.
Then I compared them, distilled the technology and also the amount they were willing to pay.
I'll spare you the details, but the clear winners were Java and C#.
On the Java side of things, it's Spring MVC ( Spring boot catching up )
For the frontend, Germans and Austrians tend to trust Angular.
So I picked Spring and Angular. So far I'm more than pleased with the customers storming in.
I'd suggest you do the same for your region, I'm also convinced that your result might look totally different, depending on your country and region.
I also did a few isolated tests, for some parts of Austria and the results surprised me a lot, for example, C# was the clear winner with over 60% market share in some areas.
What I'm trying to tell you: This question is rather biased, just because you see 10 purple buses in the morning, that does not mean that suddenly the bus company changed its colors, it's maybe just a coincidence that someone just painted 10 buses purple, the only 10 purple buses in your town/city.
I never learned it in college and only picked it up for an internship but it's has definitely proved very useful.