It is Norway, where there is the contemporary presence of Bokmal and Nynorsk as official languages, but where there is a de-facto official language, Standard Østnorsk (Standard East Norwegian), spoken by the upper class in Eastern Norway, which is also taught to foreign students.
So, yes, it's possible to have a common dialect. It's just a matter of political will and a trade-off between isolationist and internationalisationist choices.