Translation:We are speaking in Dutch, not German.
Reminds me of that bit by Ylvis where he speed dates in the Netherlands and pretends he is from the Behindcorner (Achterhoek)
That means "we are speaking dutch, not german" not "we are speaking IN dutch, not german"
Are you sure about that? Because I am Norwegian myself, and I find it a lot more natural.
the meaning of the sentence with på and the sentence being said without it being more natural are two separate ideas that are not mutually exclusive. And in English, people would say "We're speaking Dutch, not German" more so than they would say "....in Dutch", most likely. It sounds like both are grammatical in both languages.