From Wikepedia: "Various derailleur systems were designed and built in the late 1800s. The French bicycle tourist, writer and cycling promoter Paul de Vivie (1853–1930), who wrote under the name Velocio, invented a two speed rear derailleur in 1905 which he used on forays into the Alps."

So, it's French....I guess you get a choice to spell it the French way or Anglicize (Americanize?) it if you like. My own preference is to use the language of origin. That's just 'cause I like language.