A note on US high school diplomas - that's not entirely accurate. A diploma plus an ACT score of 28 or greater, or a combined SAT math+reading comprehension of 1300 or greater, or four AP classes, or a minimum GPA of 3.0 with an academic track, all qualify you. Pretty much the same sort of thing you'd expect to get into a decent college in the US, really.
[http://www.anabin.de/Xml/xmlZeugnis.asp?ID=704&Land=124&...]
Note that most US high school diplomas will not get you into a German university, you need APs or community college credits. I am guessing that it would get you into a Fachhochschule[1] (again, you would need German) which grants Bachelor degrees, but is more vocational than the Universities.
[1]"University of Applied Sciences" is an abombinable translation, which tells you nothing, but it's the official translation.