Is it possible to get any certificate of language level? How can I introduce my level on some websites by duolingo? For example LinkedIn.
A Duolingo level is like Monopoly money. It does not show how much you know, all it shows is how much practice you had done on Duolingo.
If you want a certificate, find where you can sit the Polish CEFR exams, prepare for them and see what happens. That will give you something that actually means something for employers.