Which I'm finding harder to understand as I learn more languages. Yeah, every language has its differences and idiosyncrasies, but as long as you have access to docs (if not third-party resources like expert blogs or Stack Overflow) it's pretty straightforward to figure those out and be reasonably productive relatively soon.
That is: a senior programmer should have enough background knowledge to be able to be productive on any language ("polyglot" shouldn't be a big deal), and a junior programmer won't typically have enough experience in any particular language to be definitively a "$LANGUAGE programmer". In either case, whether or not a programmer already knows the language before being hired is kind of a moot point.
My current dayjob was the first time I had written any non-trivial amounts of Python, Javascript, or C# in a professional capacity in my whole career. Being productive on them wasn't terribly hard - it's just different syntax around a lot of the same concepts (and sure, there were also some new concepts - took a good while for me to wrap my head around async/await, for example, coming from a background of message passing between threads or processes or actors - but those can be learned).
Usually the choice of programming language is far less significant as a barrier to one's understanding of a codebase than, say, the actual problem domain. Writing software for, say, a warehouse tends to necessitate knowing an awful lot about how warehouses work; whether or not you happen to know a given programming language is entirely secondary to whether or not you know the difference between a picker and a packer, or between a replenishment and a cycle count. Similar deal in medicine, or education, or manufacturing, or sales, or finance, or what have you.
