Basic Trope: People associate particular stereotypes with certain languages, such as French being seductive.
Alice uses her knowledge of French to seduce Bob.
Whenever Bob is angry at someone, he screams at them in German because he thinks it sounds intimidating.
The French language is depicted as a form of Charm Person. Its use is restricted to licensed professionals.
German is the Black Speech. Children cry and plants wither whenever anyone orders a Wiener schnitzel.
Bob spent a couple of years in France as a young man and had a passionate affair, of which he has blissful memories. He still associates the language with those feelings.
Alice had a strict German piano teacher as a child, and still associates that language with stress and misery.
Inverted: Bob can't see how anyone finds French, the language of snooty waiters, remotely sexy. But the sound of German makes him think of that glorious summer he spent with Hilda, the busty pigtailed milkmaid...
Alice tries to speak French so she can seduce Bob, but she fails miserably. Turns out Bob really dislikes the French.
Bob is angry at someone and yells at them in German. Said someone is a German-speaker (or native) who responds with "Ernsthaft?"