Python, because it's easy to learn and it's super versatile. I don't code Python full time any more, but I still find a way to use it on a fairly regular basis

Python, because it's easy to learn and it's super versatile. I don't code Python full time any more, but I still find a way to use it on a fairly regular basis. It's a very handy thing to have in your tool belt, regardless of what you wind up doing in the long term, and there's a lot of literature related to learning programming that's taught with Python. Python is probably the most average language you can find, which is why it works well for your stated purpose of having a more solid foundation. Although it's not the best tool for most jobs, it sounds like you don't entirely know what you want to do, and Python is an adequate tool for a lot of different things; it can expose you to a variety of programming environments.