I haven't used Elm, but generally I think it shouldn't be that scary to use an "advanced language" if it has an even remotely decent FFI.

It might seem like horror to have to interface to external dependencies, but if the libraries you want to access aren't extremely complex then interfacing with them probably isn't going to be such a huge problem.

In some cases you might want to use the advanced language for some things but keep the outer layer of the app in JavaScript. At a previous workplace we had a particularly tricky parsing module written in Fay (Haskell subset compiling to JS).