people building "polyglot" software aren't necessarily "fluent" in all the languages used in a software project, and it's way more important that you know problem solving and programming and how computers work to make code as efficient as you can than just knowing a bunch of programming languages, that said, you should know at least one language very well, and should occasionally see what makes others languages attractive or useful

so if you know go very well, you could read code written in something different, I think you can learn a lot from reading code in an entirely different paradigm, so for instance, you could read a program written in Lisp, or Assembly, or Prolog, just as a learning exercise, you don't have to become an expert

for example, why do people program physics engines in C/C++, but do GUI in Java or Python? because each language has its strengths (in this example, performance for the engine, simplicity and expressive power for GUI)

why do languages like Prolog exist? because they are useful for things like AI or reasearch

now, because a lot of languages sort of inherit from the C language, learning C is a good way to make it easier to get used to the syntax of those languages, because it's going to be fairly similar to C, Go itself inherits a lot from C as I understand it