. is it easy for foreigners to apply to such a big school
There will usually be no problem getting accepted if you have average grades. The university runs these English language options in order to attract as many international students as possible because it brings them a lot of money.
Is there a way to convert to that scale? a model or formula?
This might help you.
dsm.usz.edu.pl/grades/
I'm afraid that teachers won't like foreign students.
The teachers will be ok. The office staff in Poland can be difficult to deal with. They don't have a problem with foreigners but they are not very efficient. However you will usually get what you need eventually but you may have to ask several times or ask a few different people!
In general, if you have any questions about applications, it's best to contact the university directly and ask them. As it says on the University of Warsaw website:
"In case of any further questions, please contact appropriate unit conducting studies or the Admissions Office (by asking a question at the IRK system or directly)."
rekrutacja.uw.edu.pl/en/how-to-apply/