I said to one company from 3500 to 4000, but then I read on internet, that in this company they pay much more.
That would be pretty near the tippity-top of the range for the jobs you listed above.
And, as InWroclaw said, that translates only to 2000 to 2500 PLN a month netto. Hardly tempting for a foreigner. For comparison, a studio apartment in Wrocław will cost you about 1500 PLN a month all inclusive, and with less than 1000 PLN a month for other expenses combined, you will be living like a student. I highly doubt they would offer you much more, if anything at all, and I would not count on getting a substantial raise after you start. Poland is an employer's market, not an employee's market, which is why so many Poles seek employment abroad, while very few people from Western Europe come to Poland to work. I advise you to rethink and seek employment elsewhere.
And what do you think about changing the expected salary and saying a bit higher one? Is it somehow possible?
Of course it's possible, and that's exactly what you should do. Ask for 5000 PLN, and don't be disappointed if they turn you down.
Like saying, that for the beginning I would be ok with 3500, but after some time and getting more serious and responsible tasks, I would like my salary to be higher.
That would be incredibly stupid. Once you're hired, they have very little incentive to give you a substantial raise.
I speak foreign languages and I have a master degree.
So do thousands upon thousands of unemployed young people in Poland, and they would be willing to work for half of what you are asking. There is nothing you have said that indicates that you are particularly valuable on the Polish job market, other than being a native speaker of Italian, and that doesn't translate into serious cash. Unless there is something that you are not telling us.
I would just like to have a real idea on how much should I expect for these roles. Especially for an executive assistant of bosses. It sounds a demanding work, but I have no idea about the salary.
Like I said, 3000 PLN butto would be considered very good pay in Poland for any of the jobs you listed. Anything more would be extremely generous. Like I said, there are tons of Poles who would take these jobs for a lot less.