There's a difference between slang and regional expressions, isn't there?

Yes, but slang can also be regional.

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My mother's New Jerseyite family go "down the shore" instead of "to the beach". I'm sure they have other peculiar expressions, too, but I don't see them enough to recall.

A lot of people here [Texas] will tell you "leave it set" there awhile instead of "let it sit" if they want you not to disturb something.

And "copacetic" started in the 1920's (but may not have been in wide use until later) and is supposed to mean roughly the same as "it's all good" does today: If things are going well, it's "copacetic". I could see it being used to imply "in agreement" since, if you both agree on something, it's "all good" between the two of you.