When I first arrived in Denmark, it was common to attend a meeting in which Danes, Norwegians, and Swedes would each speak their own language and simply assume everyone else understood.
It was a proud expression of Scandinavian solidarity.
Unfortunately it never worked very well, and everybody ended up understanding roughly 80% of what was being said. In particular, the Swedes and Norwegians struggled to puzzle out spoken Danish.
These days, the addition of other internationals to meetings means these inter-Scandinavian conversations are now conducted in English, to just about everyone’s relief.