I'm not very good at Polish
So, please, don't call yourself "Polish" if you can't even speak the language.
I would not advise that because Polish Universities have a different diploma system that's not always recognized too well by US employers.
Wrong. EU countries and quite a few more have all followed the Bologna Process, so one Bologna diploma is much the same as another in terms of how the system works.
College educated Poles know English.
You'd think so, but I know plenty of them that don't.
I was just curious as to how I might be treated by native Germans if/when I go to study at a German university.
You can't speak Polish, what makes you think they'll treat you as anything but just another American who went abroad to buy her education?