I have an MVC4 project with language selection:
1 main part with:
Not sure if this hits the solution from a different angle to the accepted answer, but I found that one of my controllers in the Areas section was sitting in the wrong namespace. Correcting the namespace to:
Areas.{AreaName}.Controller
fixed the issue for me.
I suspect the key factor was to have all the controllers within a given area share the same namespace.