Working for the king of Prussia is one of these hundreds of sayings that exist in French like there are as many ones in about all human languages. That one isn't particularly uncommon, but Ned who's fluent in French may not have met it yet because it's not used that often and also isn't one that is known and understood by all French speakers.
All these sayings and expressions - let alone proverbs - form some sort of meta-language that gives any society its pure cultural identity, beyond the sheer technicality of the vocabulary and grammar its language is constructed with.
Am I allowed to draw a comparison with the shibboleth of the Hebrews, a word whose pronunciation serves as a definitive marker to distinguish those who are in from those who are out?
But where the mere pronunciation of the word "shibboleth" suffices to detect a foreigner, it is the very usage of those sayings by non native speakers of any particular language that indicates what could be termed as an illegitimate attempt to break the barriers.
Let me explain: Unless one is completely bilingual, everybody has a certain distinctive native accent when speaking in a foreign language. No need for a shibboleth test, the intonation is here to hear and tell the rest of the audience that a foreigner is speaking.
But when said foreigner employs sayings that are typically associated with a certain cultural group, there is an instant alarm that is set and informs that a foreigner is trying to penetrate the core of a group which cannot recognize him as a full fledged member. I guess it works the same in all societies, be it an Italian speaking Finnish and using specific Finnish expressions because we're all seduced by the mysteries of foreign languages, or a Chinese speaking Japanese (no wait, there are some other problems in that case...).
There must exist some specific Texan sayings (when the cows come home?) that some one from Illinois (say) may want to use just to show h/h knows. And although it is spoken in English and with a different accent, wouldn't the Texans be tempted to say to the imprudent intruder: "Sir, will you please keep yourself to yourself and even return to where you belong while you're at it?