Surely they wrote some papers in English even if they wrote their dissertation in German? Most people don’t go straight to dissertations anyway, it’s more of a place to go after you read a much shorter paper.
[1] https://people.idsia.ch/~juergen/SeppHochreiter1991ThesisAdv...
They wrote the paper in German, so honest people who don't know German can't cite it directly. This is only even worth remarking on because Schmidhuber is annoyed for missing out on those specific citations.