But why would you choose to get a masters in English at a Polish university?
Indeed. If your goal is to make a career in translating, you don't need a degree in English, or any other language. You just need to know both languages very well, and, more importantly, be rather well versed in the subject matter to be translated.
The best paying jobs go to experienced native-English speakers who have a background in a specialty field like science, medicine, law, finance or technology, or have tons of experience in writing for the marketing and advertising industries. In any case, you need to be experienced in writing publish-ready copy for academic journals and grant proposals. Other types of translating do not pay well at all.
I'm a pathologist and clinical research scientist, with a background in industrial and environmental biology. During my time in Poland, I translated over 500 scientific articles and 10 scientific books from Polish to English, mostly in agriculture, botany and medicine. I had practically no competition. If you don't have a specialty field, then the best jobs are beyond your reach.
I often advise engineers that, if they want to make decent money and have a fun and exciting career, they should go where R&D dollars flow in wide rivers. The same with translating. Become proficient in the language of the well-heeled sectors of the economy. Making money is a lot easier where there is money to be made.
If you want to improve your Polish, then read your butt off. During my first three years in Poland, I spent forty or fifty hours a week reading Polish like it was going out of style. It was my default state. Plenty of books that you can download for free from the internet.
As for studying in Poland, you are going to get a lot more out of it if you study in Polish. Courses taught in English are nowhere near as good. Get your Polish up to snuff by reading like a maniac first, and then revisit the studying-in-Poland idea in a couple of years. Put aside a nest egg so that you can study and live without working. There's no hurry,