Languages like PureScript provide a much more convenient FFI for this glue, but the glue and escape hatch is still there with Elm.
F# and C# compile to the same intermediate language, are interpreted on the same runtime, and are produced by the same organization for the same IDE... Their libraries are compatible and they rely on shared access to the common .net ecosystem.
Your absolute worst case scenario would involve decompiling your F# source as C#. More realistically you would use dependency management to gracefully transition out of one of the languages.
Transitioning piecemeal works exceptionally well: it's exactly how I've transitioned numerous projects away from C# onto F# ;)