I have years of experience of correcting, revising and re-writing translations from a variety of sources. Many were translations by Polish translators who were not native English speakers, and the English was poor. In such cases an enormous amount of effort is required to correct grammar and syntax and to clarify meaning. This requires the ability to re-write distorted sentences and to replace inappropriate words or phrases without losing the original meaning. Some texts were written by native English speakers, including professional writers, but the same abilities were required, even if the corrections were less radical. In both cases, the proof reader sometimes requires an almost intuitive ability to 'second-guess' the author's meaning, particularly with regard to technical matters.
Polnisch 
Englisch (Muttersprache)