I have coworkers using Rails for the back end of their Elm app. I think NoRedInk is using some Rails and some Haskell. I've have a friend who uses Python for his back ends. Also heard that some people really like using F#, as well as Node.

Or if you want to do Elm full stack there's https://lamdera.com/ or https://package.elm-lang.org/packages/choonkeat/elm-fullstac....

We're using Elixir on the backend at Corvus Insurance and it's quite nice.

Any back end stack that can provide APIs, we're using .NET.

we are using rust on the backend

We use Kotlin