its fairly straight forward LiR...
I can understand if you say that people have certain similar facial characteristics as other people of the same nationality but to come out with normal human traits that can apply to any nationality (and it does) is, in my opinion silly.
Stubborness is a human characteristic and doesn't relate to which Nationality you come from. Labelling Polish people as stubborn is stupid in my opinion.
No matter where you have lived and what experiences you have had, you have only met a very small percentage of that population and it may well be you came across some e.g stubborn people in a certain country but that doesn't , in my opinion, make that a trait, that is pertinent to all the people of that country.
and when i say 'you' up there i mean 'one' it's not personal :)