Before I am criticized for sounding too "subjective", "opinionated" or "etymological", take a look at this question:
"Was Arabic the "Lingua Franca" of the Middle Ages?
If you look at the linguistic map of the Middle Ages, you will see that the Arabic language spread from the Arabian peninsula westward, primarily to North Africa, Spain and even Sicily-(for a few centuries). The Arabic language essentially replaced or pushed aside the older Syriac, Phoenician, Coptic and Berber languages.... (confining some them to secondary linguistic status).
Of course one could make the case that Latin, was the "Lingua Franca" of Western and Northern Europe during the Middle Ages; though Medieval Latin was more of a scholarly and academic language during that time, whereas various Romance and Anglo-Saxon languages were more widely spoken in the towns and cities across Northern and Western Europe.
Yet, Medieval Arabic, was an academic, religious, commercial, political and socially communicated language across the aforementioned lands and countries.
Having said that, was Arabic the "Lingua Franca" of the Middle Ages?