Learning a new programming language - which?

I'm not entirely happy with my career progress in IT, as a developer, and want to use my free time to skill up and
In my current job, I use SQL on a day to day basis so that gives me an opportunity to build my skills there.
Although I don't use it currently, I have worked with C# in the past when building/maintaining APIs and MVC applications within a .NET environment.

I've shied away from JavaScript for too long now and given it's versatility and popularity, I feel this would be a good use of my time. I've not really done much front end development work so I think JavaScript would be a good language to learn in that regard.

What are your thoughts?

I would personally recommend either Java or Python. (I'm kind of surprised that you haven't worked with those yet.). Python is excellent because not only is it easy to learn due to its simplistic syntax and powerful properties, but also it's a general purpose programming language that is especially popular for fields like machine learning and data science. When I left university I didn't want a boring web developer job and I wanted to do something that's a bit more interesting like AI, ML and data science. For my day to day job I use python for data analysis and it's incredibly fun because I get to explore large datasets and create interesting visualizations. There's so much that you can do with python and it can open a lot of doors for you.

The second language that I would recommend is Java. It's also a general purpose programming language. I don't like it as much as python, but it will also open a lot of doors for you because a lot of companies use it. When I was looking for jobs, I would say that Java was a popular requirement from many employers. (alongside C# and .NET), so it would definitely be beneficial to learn Java.

Note: I am not working in IT and I am a learner myself.

It depends on what you want to get into. If you want to go into front end development, JavaScript is a good language to learn.

If you want a general purpose language that allows you to go into anything, as well as upskill as a back end developer, Python is a good fit.