Air Mail stickers often have Par Avion for the same reason.

"Taxe percue (pronounced TACKS-pair-SOO): An auxiliary marking (sometimes abbreviated "TP"). It is applied, usually by handstamp, to indicate that postage is prepaid. Taxe percue markings are often used instead of postage stamps as a matter of convenience. They are also used when postal rates change rapidly and stamps in useful rates are unavailable."

French pops up in all sorts of odd places. English is often described as being the lingua franca of the internet.

Lingua franca meaning "French language".

Maybe there's a world governning body for postal services and it's headquartered in some french speaking country.