”Underpants” is commonly ”underkläder” but we often specify what kind as well ”kalsonger” (boxers, briefs, men’s underpants) or ”trosor” (knickers, panties, women’s underpants). Notice that trousers is also accepted for byxor though and it is the word that shows up for me in this discussion thread.
I'd like to add that the course is supposed to be in American English, so it should be the pants (there may be inconsistencies though, feel free to report them). I guess we'll just have to add some underwear words as accepted translations. Some of the sentences will be more fun that way too :)
The headline is wrong at the moment, but I'll see if I can fix that.
The course does need a default, and Duolingo HQ defaults to American English. Other major regional variations are accepted throughout the course, though. If a specific word or phrase is missing, please report that.
Having a separate option to always default to e.g. UK spellings and word choices is not something that Duolingo offers, and it would be a major change to the system so I can't see that happening any time soon.