Personal experience: Once sat across from three Arab teens on a subway, and they were talking a blue streak in Arabic. Didn’t understand it, because it was dialect, and I only know literary, but their tone made it obvious they were ridiculing me. Then one of them noticed my 2nd year Arabic textbook, and the one word I understood was him saying “hua yafhimunaa,” which means “He understands us!” You could have heard a pin drop.
And then the pinhead was silent...
Revealing story which shows their true colors.
Separately, please see my post # 63.