I'll echo this - when I've worked in polyglot shops, specifically on a team that did a lot of Erlang, we hired for "exposure to functional paradigms", not Erlang. The only places that looked to hire the specific language (and even that was negotiable) were places that just had Java. And the average quality of applicants was universally worse.
It's a common practice to at least say what stack you're working on on a job ad. If it's not a requirement I usually see "We use Rails but don't expect you to know it already".
The JD was, obviously, far more descriptive than the four words I listed there. I'm saying the actual, relevant requirement, not the description of the job.
