Undewear or panties would be acceptable but not pants, for that would mean trousers
Pants and underpants are the same thing people wear under trousers or a skirt.
I don't think so. Underwear is too inclusive, meaning both underpants/pants and undershirt/vest. I believe chupi refers specifically to the underwear worn around one's loins, i.e. underpants/pants/panties.
In the Free Dictionary (online) it gives the definition of "underwear": clothing worn next to the skin under outer clothes. So when DL doesn't accept "underwear" as a translation of "chupi" for once they're being consistent with standard English. They seem, in most cases, to be partial to American English which is disappointing in that the course is made available to an international audience:) But thank you for the feedback. It contributes to the widening of our language and cultural horizons! You deserve a lingot or two:)