More than one people use the anonymous account, so I won't really sacrifice my anonymity by sharing. That said, I'm Bulgarian and we use both "ножица" (scissor, singular) and "ножици" (scissors, plural), both deriving from "нож" -- knife; the latter also means more than one pair of scissors. We never use "a pair of scissors". Unfortunately, I couldn't see if the singular or plural form came first, because the very first published Bulgarian dictionary from 1900 lists both. It also doesn't have the word for "pants", so I can't comment on that neither. In general though, we never use "a pair of" for things that are physically a single object, like a scissors or a pants; "a pair of scissors" would mean two distinct scissors. Of course, all languages have exceptions to every rule and for the above in Bulgarian it's "glasses". We do say "a pair of glasses". We also say "a frameS for glasses" speaking about a single frame for a single pair of glasses.