I think our current GSD, Jaeger, understands quite a lot! He seems to know what we are saying, and sometimes acts on what we say before we are done talking, lol. We have even tried not using any emotion, or names,or any gestures of any kind and he will still act on it.
It depends a lot on the dog, and whether they are attuned to you I think, so it matters to them what you are saying. Then they pick up words and actions, and probably your tone in some cases.
But I have seen some amazing things from all the GSDs we've owned, and some were just better at it than others, lol. We had one who would go get the items we told him to get, by name, and he was not trained on those items. He just seemed to know, and did it. He also seemed to understand exactly what we wanted, without specialized training of any kind.