Updated Polish translation. Fixed typo in english texts.

[kwiateusz]

Hi, I've added missing polish texts. And during that I found one typo so i fixed it too.

I hope it's not problem to translate language name in locales.xml? If is I can revert that.

[markusfisch]

Thanks a lot 👍 And, no, fixing the names in locales.xml is not a problem. Well appreciated! 😄