What's the plural of "lingua franca"?

Wikipedia (http://en.wikipedia.org/wiki/Lingua_franca ) uses both "linga francas" and "linguas franca". But if the term comes from Italian, does the plural in Italian would be "linguae francae", as Merriam-Webster says (http://www.merriam-webster.com/dictionary/lingua+franca ), as if... show more Wikipedia (http://en.wikipedia.org/wiki/Lingua_fran... ) uses both "linga francas" and "linguas franca".
But if the term comes from Italian, does the plural in Italian would be "linguae francae", as Merriam-Webster says (http://www.merriam-webster.com/dictionar... ), as if it were Latin?

And to finish, how should the plural be in Spanish? ¿"Lenguas francas"? In Spanish I always use "lingua franca" and not "lengua franca". In that case, should I go with "linguae francae" or with the Spanish version?
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