their kids speak fluent German and have been in the German school system
In the last few years I've had at least three students who've spent significant portions of their lives in Germany (and often speak German a little more natively than Polish). And I'm not in a department that would naturally attract them so I'm assuming there's more out there.
The Germanness comes out in different ways, in one it came out in her accent (clearly German in any language) though very Polish in dress and body language while another was super German in dress and body language (but spoke Polish without any kind of accent).