Finally enough, the French expression is “présence d’esprit”.

Only few people in France know about this sequence involving Diderot and the king, and I only know about it because I’ve lived in… California.

Actually, "esprit d'escalier" is indeed used in French, though less commonly, especially outside of literary contexts. It refers specifically to the inability to come up with a timely response or comeback (and the frustration of finding the "perfect" reply when the conversation is already over). On the other hand, "présence d'esprit" has a broader usage in everyday language. It can mean having a quick and witty response (the opposite of "esprit d'escalier"), or more broadly, the sudden ability to judge and react appropriately in a situation.

edit: typo