3) In my years of copyediting—academic books, biographies, fiction, magazines—my secret weapon has been my ability to deal with text from other languages in an English context. This includes languages I couldn't speak a sentence in. How? A bit of basic knowledge and resourcefulness goes a long way…

To be honest, they are all intereting and perhaps all need addressing. But which first depends on your audience, especially among professionals in their fields. Perhaps more people would benefit from 1. than 3., but that's just a guess.