I’m assuming that you mean Monchengladbach, but I’m not sure that German fans are the best example.
Yesterday, my seat was on the segregation barrier, but the only nastiness originated from the City end, when Swansea equalized. Nothing too vile, but a bit embarrassing watching grown men impersonate hypocritical idiots.
Last season, I attended the Cologne-Monchengladbach derby, which according to German sources is only just below the Dortmund-Schalke rivalry
There were dozens of Monchengladbach fans in the Cologne end, all wearing colours, but there was never a sense that things would get out of hand.
Six weeks later, there were hundreds of Monchengladbach fans in the City end, which seemed to take everyone by surprise. They all went-up when BMG scored, but they were hardly taking liberties. Yes, it got nasty for a few seconds, but only because a few City fans were incapable of counting to ten.
However, Celtic is another level, so I agree with your overall point.