@Tamerlane I mean it's an English translation, so the translators may as well have chosen a word which just conveys the meaning and not literally means the language of the Franks.
@Tamerlane From Wikipedia: "A lingua franca also known as a bridge language, common language, trade language, auxiliary language, vehicular language, or link language, is a language or dialect systematically used to make communication possible between groups of people who do not share a native...