Nothing to do with American spelling and not a 'nose-up' to the British. French has been the language of diplomacy since medieval times. Postal use of the French language relates to French influence in Egypt from their period in possession prior to the British and Ottomans and the later presence of French Post Offices in Egypt. The French language was used on Egypt's stamps during the following periods and in the following forms;
1879 - 1914:
Postes Egyptiennes
1925 - 1956:
Royaume D'Egypte
Postes D'Egypte
Egypte
Republic D'Egypte
This is a good place to start for information on Egypt and its stamps;
http://www.stampdomain.com/country/egypt/Terry
Edited for additional information. TC.