This is not how we say hello..

Hæ hæ! :D

what what what

The joke is that the Norwegian didn't quite catch what was said, and replied "what?" (hæ?), it's also common for us Norwegians to reply like that, out of reflex, to almost anything

its a common thing in america as well to the point there are tips about getting people to stop doing that in response to stuff you say. A common one is to just not respond after they say "what" as their brain will eventually kick in and close the silence with their actual response to the thing you said.