[gohrt]

This book is written by 2 Chinese -- who may well be much more capable in English than German.

It appears that other books in the series are written in German: https://kops.uni-konstanz.de/browse?value=Universit%C3%A4t+v...

[zhemao]

I was assuming that the authors themselves had gone to grad school in Germany, so they should be proficient in German. Maybe they would be more comfortable in English, but since part of their audience is German, I would think they'd make the effort.