As a developer, it is important to be working in programming languages that are more in demand but more important is an area within the language that makes your expertise more niche than broad.
It’s not about what you can learn and which programming language has better statistical numbers but being better at what you are doing provided it has opportunities that you can be an expert for.
As an example PHP may have more freelance opportunities than Ruby on Rails or ASP or C# but if you are a C# developer, you should not focus on how many jobs are available in PHP and think about switching from C# to PHP but focus should be to become niche C# freelancer provided the niche in C# has ample opportunities.
Don’t opt to learn what is more in demand but it is about learning more what you know provided it is large enough niche for you to be able to find good opportunities.
So I don’t recommend new programming languages or specific tools that you should learn just to get more work but focus on what you are good at and try to get more specific in it. If you want to be learning something, try being a better in communication as it is the most required skill for a freelancer.
There is an exception that if you are working in a niche technology like say PowerBasic which has nothing to offer in the freelancing world, my suggestion will depend on what you want to be.
- Web developer – Go for PHP.
- App developer – Swift 3 will be my first choice but Java and Android may not be bad choice either.
So in case you are doing a move choose the one that has the most opportunity to start with and slowly narrow down your expertise. If you start with PHP, you can move to CakePHP or Laravel or WordPress.