"She wears socks."
Translation:Hon har på sig strumpor.
Yes "longstocking" translates into "[lång]strumpa". "Sock/socka" is always short: at or just above the ankle.
"Strumpor" is most common one, use it. It was a while since I heard someone say the word "sockar".
I understand that Swedish doesn't generally distinguish between present and present progressive tenses, but because of the literal sense of "to have on," it makes me wonder if this particle phrase can only mean she has socks on right now, or if it can also mean that she generally wears socks depending on context.
It is – that works just as well, no difference in meaning, and it's an accepted answer too.