Interviewees don't get to pick the language?
If you're hiring based on proficiency in a particular tech stack, I'm curious why. Are there that many candidates that you can be this selective? Is the language so dissimilar that the uninitiated would need a long time to get up to speed? Does the job involve working on the language itself and so a specifically deep understanding is required?
It can be annoying, but manageable. I've never coded in Java for example, but knowing C#, C++ and Python I imagine it wouldn't be too hard to pick up.
Regarding the job ads, yes they'd describe the ideal candidate but I haven't the experience that the perfect candidate ever actually shows up. Like you say, knowing J, T and Z, the company is confident enough that you'll be able to quickly pick up dotting the Is and crossing the 7s
For leetcode interviews, sure. Other than that, at least familiarity with the language is paramount, or with the same class of language.
Every time a new frontier LLM is released (excluding LLMs that use input as training data) I run the interview questions through it. I’ve been surprised that my rate of working responses remains consistently around 1:10 for the first pass, and often takes upwards of 10 rounds of poking to get it to find its own mistakes.
So this level of signal to noise ratio makes sense for even more obscure topics.