It depends on the things that you want to learn and amount of extra work you want to put into it. For instance, I used Go for the most of AoC 2018 problems and

It depends on the things that you want to learn and amount of extra work you want to put into it. For instance, I used Go for the most of AoC 2018 problems and I had always been using C/C++ in algorithmic contests when execution time mattered. And yet, I wouldn't say I know any of these languages good enough to write a production-grade system because I only learned usage patterns common for small, write - run once - throw away applications. On the other hand, the author of that post seems to have used a lot of more complex language features, did profiling, debugging, made use of many libraries etc - a lot of extra stuff that wasn't necessary to give correct answers but is crucial in 'real world' programming.