I never really "got" Polish jokes. There are a few that make sense and those are fine, but the bulk of them you could basically swap in any nationality and they are still the same joke and equally stupid, suggesting that "pole" was chosen arbitrarily. There's nothing specific to the Polish nationality in them.
Notice how it says "enters the bar" instead of "enter the bar".
It implies, before the punch line, that they are the same person.
Whoever made this has good grammar, since, without at s at the end of enter, it would sound off.