"It is you."
Translation:Det er jer.
I understand what you mean, but you haven't explained it very well here.
I think the difference is that, in the first case, you are asking about an attribute or identity aspect of someone. Are they okay? Are they Marxists? Danes?
Whereas, in the second case, which is what this particular question deals with, you are actually seeking to identify whether a person or a group of people is / are the person / people you are speaking to.
You got the German wrong, btw. The verb would agree with the other subject there (which would also, likely, get moved to the front):
Ich bin es, Du bist es.
I would additionally argue that “It is I” is incorrect English on the basis that although grammar guides suggest it (on the basis of Latin grammar, which obviously shouldn’t be applied to English…), English has never worked this way in actual speech and probably never will.
French has a very simple way of explaining this. They call them stress pronouns. In my view, that is exactly what is going on in the English and the Danish, as well. The object forms somehow have a weightier-ness that the subject pronouns don't have which makes them more suitable to be used in this way.