Ask HN: Languages in Interview Questions?

So I'm interested to see how common this experience is and how others deal with it when it does occur.

You are interviewing for a job where you don't know the hiring company's stack (in my case, I don't know Java yet), and the recruiter/hiring manager says "No worries, use $FAVLANG for the code quiz". You sit down at your computer and log onto the site they're using for technical interviews, and the language choices are locked down to Java 7/8, with $FAVLANG being conspicuously absent. Oh and the 2 hour timer just started.

This has happened to me twice now at two separate companies (both Java curiously enough). The first time I tried to cram the docs (reading Java is easy enough, how hard can it be to crank out an interview question?), but between syntax errors/not knowing the APIs and fine grained semantics (ex is it destructive or not?) it turns out to be really hard to crank out even a relatively simple program in a language you've never used in anger. The second time I just shot them an email giving them a heads up and made an answer in a language they had verbally confirmed as OK.