This surprises me as I've never worked on a nontrivial system that's also a monolingual system.
There's nearly always another language to use to get the job done, whether for configuration, scripting/automation, or because backends and UIs are written in different languages, especially for internet-based software.
Sure, but in most cases programmers spend most of their time only in one 'part' of the system and only really have to know the language that part of written. In general it doesn't matter if the back end server is written in Lisp, Java or APL, as long as it talks to my part of the system in the way it's supposed to.
Perhaps that's a reflection of the structure of the organizations they work in.
Anyways, the other cause of multilingualism is switching jobs, projects, or problem spaces. All three of those often involve a language switch.
No, that's not it. It's that if you can only learn one programming language, Go is not a very good choice, so those who learn Go are almost always multilingual ("lingual" as in programming languages, not natural languages). Python and JavaScript are Swiss Army knives with outstanding ecosystems that make them excellent choices if you can only afford one tool. There are plenty of reasons to learn Go but, as a more specialized tool, it makes a better component of a nice toolset than a solitary multitool.
My anecdotal experience says that 1) most programmers are monolingual, 2) almost all Go programmers are multilingual, and 3) the average multilingual programmer is a better programmer than the average monolingual programmer for various reasons, so the average Go programmer is better than the average programmer. And, by that logic, the average Rust, Haskell, Erlang, Lisp, etc., programmer would be, too.