From http://merriam-webster.com: 

Etymology: Italian; literally, "Frankish language"

	1 (often capitalized) a common language consisting of Italian mixed with French, Spanish, Greek, and Arabic that was formerly spoken in Mediterranean ports
	2 any of various languages used as common or commercial tongues among peoples of diverse speech
	3 ''something resembling a common language''


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It is somewhat ironic that English is the ''lingua franca'' in many parts of the world and in many endeavors, yet uses this foreign phrase.  (On the other hand, part of the reason for the success of English may be the ease with which foreign terms are adopted into English; see PurityOfEnglish).

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I have heard that French is the lingua franca of France.... 

''Note that "lingua franca" refers to the old language of Frankish, not to French.  Though the two have much in common; they are not the same language.  It is incorrect to think that "lingua franca" is a reference to the global status that FrenchLanguage once enjoyed, and still does to some extent.''