[stan_rogers]

The lingua franca wasn't "the language of the Franks", it was an elaborate pidgin used for Mediterranean trade that encompassed a lot of broadly Romance features with a vastly simplified grammar and a vocabulary core that could be described as most closely resembling a mixture of Occitan southern France and Savoyard (which are both, admittedly, umbrella terms that incorporate rather a large number of dialects). The term lingua franca has gone on to mean any (usually simplified) language that's used to communicate between groups - like Swahili in large parts of east Africa (it is by far the simplest of the Bantu languages, and has only a relatively small population of native speakers).