I remember once seeing a thread on Reddit, either on r/YUROP or r/europe can't really recall which of the two, where they banned English for the comment section of that thread and invited all Romance speakers to speak in their native language. It was surprising how anyone could pretty much understand and hold conversation with everyone else. Granted, it's likely that written makes it a bit easier, compared to spoken, but it was very fun!
I suppose we could also hold something similar for Germanic or Slavics, if a sufficient number of those speaker were down to try this experiment