A term being derogatory doesn't matter if it's the correct translation, that's not an argument. Your second part is more convincing. In absence of a direct English term with the same meaning, 'online crossdresser' with a translator's note in the margins about the term 'nekama' could be better, though 'tranny' still conveys the idea just fine and is more succinct.A panel on page 10 was translated improperly, "tranny" is a very derogatory term, and after looking into it, the direct translation is nekama: a person who portrays themselves as the opposite sex online.