Someone maybe Charlemagne said he spoke French to his mistress, German to his soldiers and Spanish to God.
I won’t verify accuracy, but I did find this on the internet:
“Charles of Ghent Speaks Italian to His Mistresses, French to His Courtiers, German to His Soldiers, Spanish to His Churchmen, and Dutch to His Horse”
I worked for a couple of Egyptian brother and one told me that they spoke English for business, Arabic if they were discussing personal matters, and French for more intellectual discussions because French was what they had learned at school.
Several times I heard them discuss a single issue in all three languages.
The brother who was the boss was really brilliant, and he could speak Italian too. (And who knows what other languages, he was nutty but super, super smart). I once heard him have an hour long discussion with an Italian vendor (who only spoke Italian) filled with a discussion of pieces, prices, all kinds of numbers.