Почему южных? Вроде в 4 книге стоит "Slytherin from Fen". (с лексикона)
As mentioned above, the Sorting Hat tolds us that Salazar hailed from "fen" or the marshy areas of eastern England. (lexicon)

Пояснение на вики

The Sorting Hat described him as "power hungry Slytherin" and that he came "from fen."[4] This could be the reason why he and his family have the rare gift of speaking Parseltongue. It is also said that anyone who can speak Parseltongue is somehow related to Salazar Slytherin. The fens of Eastern England are in Norfolk, Lincolnshire, Cambridgeshire, and adjoining counties, although it is not established that Slytherin actually came from that particular region.