Since the Republic of Poland is a member of the EU, it shares the common EU educational system.
Yes, but like most of the EU, these things are only superficially the same.
That means that university transfers between European countries are possible, right?
In theory. The reality is different, due to courses being designed in different ways.
I technically finish the German university, so I should end up having the German diploma
It won't work like that. Many universities have limits on the amount of ECTS points that you can transfer, and you can also expect them to check in detail as to whether the modules in Poland really matched the German one. I absolutely guarantee that you won't get everything transferred.
In short, your idea is nice, but Europe simply isn't integrated at that sort of level to allow that kind of transfer.