Maybe I’m just dense, but I certainly have not been productive with any language on day 3. (Not even Ruby, which feels comfortable faster than most languages.) I sometimes felt productive that soon but I always end up having to rewrite once I learn the language fully.
I’ve learned enough languages that I can spot “a person’s first program in X” now pretty easily.
My background is freelancing/consulting where it's not uncommon to gather diverse individuals to work on a greenfield project.
There's an implicit assumption that all participants know a thing or two about programming, and accordingly are apt learners.
So yes, on day 3 the code won't be perfect, neither on day 6 but after a few code reviews one can get stuff done.
Personally I wouldn't be comfortable prescribing Clojure for such a project, in contrast to other languages which I've never coded in, but that I know that socially, today, work better.
I’ve learned enough languages that I can spot “a person’s first program in X” now pretty easily.