I would say many Southern dialects are (or were, more exactly) proper languages, it's only with the advent of the Unification of Italy that the "florentine language" got the upper hand.

For example just this evening I was listening to this Neapolitan (I think it's Neapolitan, definitely Southern) song Brigante se more [1] and I needed to see the written down lyrics to get a hold of it [2], as I couldn't understand almost anything at a first hearing. I know Italian pretty well, but I just couldn't parse the song when hearing it directly.

[1] https://www.youtube.com/watch?v=haF2sPzb63s

[2] https://theitaliansong.com/songs/brigante-se-more/