I was surprised this isn't the most popular answer. IMO programming a complicated game such as a roguelike can get you a very well-rounded knowledge of a langua

I was surprised this isn't the most popular answer. IMO programming a complicated game such as a roguelike can get you a very well-rounded knowledge of a language. But what makes RLs absolutely ideal for learning a language is that you will get your first version running with an extremely minimal amount of code (and no other resources), and you build up from there. It's very strongly motivating.