The mailman.po file looks OK to me and compiles fine, so what can be the reason?
As there was no response so far I'm wondering if this an extremely difficult question and/or if there's any better place to ask?
I also noticed mixed languages response in various situations regarding Polish language. Also in the list-owner administrative interface when set to Polish some text is in Polish, and some in English, and what's more interesting, sometimes it can be in English, and the next time it can be in Polish. Maybe it depends on the phase of the Moon or strength of the wind or something... ;)
Cheers! ak
PS. Who did the translation to Polish? As there are some style errors. Like sentence shouldn't start with "Aby" because "aby" is conjunctive. I found also one mistyping.