I think this would have been more true 10 years ago when Haskell and Lisp were the most likely place for people to have encountered "let".
Now? It's in Javascript in its ES6 evolution and related languages, Swift, Rust, and probably other "newer" languages I'm not aware of. Scala had it ten years ago also, but is more mainstream than it was then.
30 years ago I think it would have reminded people more of BASIC or Fortran than (say) Scheme.