Thanks for the correction, I was indeed wrong (parts of my comments were based on true facts but parts were clearly wrong, which is harmful). Honest mistake, I'm glad you spotted it so that I avoid propagating it in the future.

To be dishonestly picky, French is somehow etymologically the "language of the Franks"... However, from what I just read, while "lingua franca" designates a vehicular language (which is the main thing to remember about the expression), it was originally used to refer to a simplified mix of languages (Portuguese, Spanish, Italian, French among others).

Thanks again, I guess I made long ago a lazy intellectual shortcut conflating my somehow credible explanation with the actual origin of the expression.