"Es mi pantalón."
I always thought that the quality of translation shouldn't be in translating literaly but with understanding and with knowing phrases and use of words in different languages, because these things are different, you just can't translate all the sentences exactly as they are if you want the best translation...
They are my trousers. OR They are my pants. are really the only correct translations. It is my pants is not used. El pantalón = a pair of pants (or trousers). The word trousers is used more in Great Britain. It is a little old fashioned in North American usage--and certanly only used for men's pants not womens.