How so?
Because lingua franca is Latin for "French language"
From Wikipedia:
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:
Not to be confused with French language.
It would actually be closer to free language.
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.
