Most of the slave characters in Scandal use dialects; does this mean they were sold from the provinces?

‌Or
did the writer simply make a distinction where
standard language represents the nobility
and dialects represent the slaves?
Of course, that is a possibility... even in Bukchon..
Since slaves were essentially commodities of that era,
as Chil-seong said, owners change seven times or even more,
so we cannot rule out cases where they came from the north or south to Seoul.
We can't assume that transactions only happened locally.
Still, since the prominent slave characters in the drama use regional dialects,
it makes me wonder if it's some kind of regional disparagement..