The relationship between the French and the English has always been complex. We just love to hate each other. The English have countless offensive words to describe the French, and the French have just as many to describe the English.
Invading each other is a favourite sport. It began with William the Conqueror (pardon me, Guillaume le Conquérant) and it is still goes on, with young French professionals taking the streets of London (many of my ex-Parisian friends have moved there), while senior English retirees settle in Brittany (Bretagne) and Périgord.
In language, the same has occured. The English have borrowed many French words, and now the French are (mis)using many English words. Well, American words now. Sometimes it backfires. For instance 'tennis' is an English word used by the French. But the English word comes from the French 'tenez' that was used in the jeu de paume.
Why am I writing this again ?
Oh yes. 'French leave' is translated in French into 'filer à l'anglaise' (English leave).
Thanks to isogolem for correcting my English.