You know that whole thing, like, “confronted with a foreigner, dude continues to speak his own language, only louder and slower, ha ha what a fool”.

Except that’s not a terrible instinct, when you consider that through most of history, the foreign languages you were most likely to encounter were the related dialects of the next region over. Like, carefully enunciating English won’t help a Mandarin-speaker understand you, but it might be useful to a speaker of Dutch or even German. In the other direction, consider how English-speakers can kind of follow Jamaican patois if spoken slowly and clearly but get completely lost at conversational speed.