FYI - I Spilled Battery Acid All Down The Legs Of My Pants, Once.
They were some kind of polyester or synthetic fabric, made from cloth that probably was actually comprised of “plastic” fibers. Whatever the explanation, those pants were not harmed by the acid! They lasted for years. Apparently my pants were similar in composition to the plastic container that the acid came in or the stuff from which the battery case was made.
I have used carburetor cleaner to remove stains from fabrics and even car upholstery. If your seats are “cloth”, there’s a chance the fabric won’t be harmed.
The carpetting could be OK, too. I can’t speak for the seat’s foam, springs, or the floor of the car, though.
I’d follow everybody’s advice for diluting or neutralizing the acid, but the extent of damage could just be a “wait and see”.