Here's an advice: browse for job posts you'd be interested in. Look at required/preferred qualifications. Start learning those. When ready, apply for those kinds of jobs.
Also, find out what other libraries/API they might use. For example, the company could be using some niche software that's nice to know, but it wasn't important enough to list in the job description. The HR people won't care if you say that you know X, but you might impress a fellow developer during the interview, and that's a plus.
This is, from my perspective, the only advice that matters when discerning what to learn while seeking employment. Look at businesses and people that you admire and see what tools they're using and see what they're looking for. It's one thing to see that there are a lot of (potentially boring and poorly paid) jobs out there using some technology and another to see that you can do the kind of work you want to and find a job you'll enjoy if you learn something else.
This is actually excellent advice. It will also drive you to recognize what you need to be able to demonstrate to actually get the job after you become proficient in the language / framework you select.
Also, find out what other libraries/API they might use. For example, the company could be using some niche software that's nice to know, but it wasn't important enough to list in the job description. The HR people won't care if you say that you know X, but you might impress a fellow developer during the interview, and that's a plus.