A lingua franca (/ˌlɪŋɡwə ˈfræŋkə/; lit. ‘Frankish tongue’; for plurals see § Usage notes), also known as a bridge language, common language, trade language, auxiliary language, vehicular language, or link language, is a language systematically used to make communication possible between groups of people who do not share a native language or dialect, particularly when it is a third language that is distinct from both of the speakers’ native languages.
Actually Wikipedia specifically has a note at the top:
In Lingua Franca (the specific language), lingua is from the Italian for ‘a language’. Franca is related to Greek Φρᾰ́γκοι (Phránkoi) and Arabic إِفْرَنْجِي (ʾifranjiyy) as well as the equivalent Italian—in all three cases, the literal sense is ‘Frankish’, leading to the direct translation: ‘language of the Franks’. During the late Byzantine Empire, Franks was a term that applied to all Western Europeans.
So if anything, it’s (roughly) Italian, Greek, and Arabic for “language of the Western Europeans”.
Yes, because that used to be the situation back then. It’s whatever language all involved parties speak. So English being a lingua franca doesn’t even contain trace amounts of irony.
In the past, in French there were zone, comté, royaumes (area, county, kingdom) franc(he)s. It meant they were free of the rule of the king.
Lingua franca translate into langue franche. Not langue franque because that would be the language of the francs. But at some point in time franque and franche probably were the same word.