Reason why proofreading is required is because direct translation from Polish to English never really makes much sense.
No. That is why translation is needed. It takes a lot more than "proofreading" to make an amateurish "translation" by a non-native speaker readable. It usually turns out as pure Polish, just with English words, and often the wrong words, instead. Stylistically, it's usually downright ghastly. A complete rewrite is needed, which often involves translating it back into Polish, and then into proper English. I often can't make heads or tails of it, especially when they use Google Translate.
50zl per page will most certainly get you somebody who will translate from Polish.
Perhaps for "business" English, but if you want a competent legal, medical or scientific translation, 90 PLN per hour is a deal. As far as I know, I was the only native speaker in all of Poland that translated scientific and medical articles for publication from Polish to English. If you can't understand the science, there's no way that you could translate at this level.