> I think many developers are productive in one language: in the one that they know best.

While superficially true, this conflates cause and effect. I am, or was at one time, extremely proficient in multiple assembly languages, Fortran, C, Pascal, Modula-2, Verilog, and Python, marginally proficient in multiple other assembly languages, Perl, Tcl, etc., and have toyed with various lisps and functional programming languages.

I viscerally recoiled from a few languages, like Java, Javascript, and C++, and found other languages, like Go, Lua and Julia, not compelling enough to bother switching to for my use cases.

And while I can certainly believe that many people are most productive in the language they know best, that most emphatically doesn't mean that they learned a language and became proficient in it to the exclusion of others. It most certainly meant in my case that I spent more time with, and learned ever more about, a language that was already productive and useful for me.