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).
Yes, as long as they are white underwear. Colored underwear/panties/boxers will bleed and lose their coloring.
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."
Pantys
its called pantiloons
Lingerie.