"They wash their trousers."
Translation:De tvättar sina byxor.
I don't think it's fair to blame the sentence… the English sentence is ambiguous, we don't know if they are washing their own trousers or the trousers of some other group of people. In Swedish, it would be sina for the first case, and deras for the latter case, so both versions are accepted, but sina is the main translation which is probably a good choice since that seems to be more likely.
Ok thank you. The whole concept of trousers coming in pairs instead of a single wearable object probably bothers me more than it should (even in english). How do you handle specific trousers, like jeans in Swedish. Can I go to a shop and buy "två jeans" meaning two pairs of trousers? (not sure about the grammar of jeans, according to google the plural is still jeans)