I've had a few people be nuisances (idc I always got my seat in the end), but the one exchange that always comes to mind is this one. Once there was a mom with two kids sitting in my two seats, who pretended to not understand English, and then started to say horrible things about me about and cuss me out to the flight attendant in a local Zambian language (Bemba, which I speak because most of my colleagues prefered it).

Cue her surprise when I as a white guy told her in fluent Bemba I've been kind and patient with her, but I don't appreciate her attitude and asked her to leave one last time or I'd have to get her moved. Suddenly she can apologize in perfect English and runs to the back dragging her screaming kids with her. The flight attendant thought it was hilarious. Wished the other lady a nice stay in my home country when we were at the baggage pick up but didn't hear anything back.