Former linguistics major here. Interestingly, 'lingua franca' originally referred to a specific pidgin trade language spoken in the Mediterranean. The 'franca' part referred to the Franks, who were originally a Germanic tribe that established kingdoms in what is now France and much of western Europe. By the late Byzantine period, 'Franks' had become a blanket term for all Western Europeans. What happened to both 'Franks' and eventually to 'lingua franca' is an example of semantic broadening.