I've never encountered a single AI or CS paper translated to or written in German. You may not need to speak English for a PhD in, say, law, but you can't be a competent computer scientist only knowing German.
As someone that graduated during the 90's, and had graphics programming as one of my main focus, there were plenty of German only papers out of Darmstadt.
We had a couple of proceedings books with them on our university library.
People from other discipline may want to learn IA. Say for example someone majoring in linguistics that wants to do some NLP. And in this case, English proficiency may not be a requirement in his curriculum. Tons of researchers did and are doing their career in Europe without using much English.
As someone that graduated during the 90's, and had graphics programming as one of my main focus, there were plenty of German only papers out of Darmstadt.
We had a couple of proceedings books with them on our university library.