Questions to ask about linguistic politeness

Not sure exactly what your question is, but my first question to anyone with a foreign accent is generally: "You aint from around these here parts, is ya, boy?" In the meantime, while politely awaiting an answer, I am slipping on my brass knuckles.

I assume the question involves how, in the case at hand, a Polish person with moderate proficiency in English can avoid impolite expressions while speaking with a "native" English speaker from the UK, Canada, the USA, Australia or New Zealand. Correct?

That's a difficult question as I assume the real risk is avoiding the traps associated with unique or unusual local meanings attached to words or expressions. In the case of English there is a great deal of variety in this area from Britain to Scotland, and in the USA & Canada from region to region. Australia is an entirely different challenge. Americans are generally more direct in their speech than the British, perhaps in the same way you referred to Poles.

Saying "Your mother has nice tits" or "Your sister is a double bagger" are probably not polite in Poland.

Ask him how he feels things differ as regards your topic in Polish versus English.

I have this strong instinct that I attend the same lecture as you did a few years ago. It's quite calming to know I'm not the only one who is completely lost with this task. May I ask what kind of questions you ended up asking? I'm doing my interview with a girl from Zambia whose mother tounge is Lozi, and I'm totally clueless...