The answer is long underwear.
Long underwear is "långkalsonger", in Swedish.
longjohns are long underwear, therefore no, wear them on their own.
Old-fashioned underwear (called long johns) used to have an actual flap in the back that covered up the butt - or that could be unfastened and let down so that you could use the outhouse (they didn't have indoor plumbing back then either).
Thermal underwear is called "thermal" because it keeps the wearer warm. It is called "underwear" because you wear other clothes over it.
Underarmour is the best long underwear, they also sell long t shirts. REI and Jockey also sell good long underwear that is good for outdoor and athletic activity.
Santa wears long underwear because it keeps him warm.
Underwear in French is called "sous-vêtements."
Lingerie.
its called pantiloons
Pantys
You can buy long underwear and long thermal underwear from online stores such as Amazon. They offer a wide range of products at great prices. You could also use John Lewis and Marks and Spencer.