How to correctly display Polish diacritic symbols in groff?

Character A0 is an unbreakable space. It looks like it is between "Jakieś" and "informacje". Use your editor to replace it by a normal space and you should be good to go.

Advice: I've set up my editors (emacs, vim) to highlight unbreakable spaces because I sometimes unintentionally type some with AltGr+space when I hit space after typing a character requiring to press AltGr.

The warnings after your first guess seem to show that some chars (ę, ś, ż...) are encoded with combining diacritics rather than natively. E.g. ę == e (hex 65) + combining ogonek (hex 328) rather than "e with ogonek" (hex 119). How do you edit your source file? You may use a Compose key to produce "standalone" letters-with-diacritics, e.g. Compose e , for "ę".