The east/west distinction probably isn't useful here beyond the observation that black mustard seeds are the hottest followed by brown and then yellow mustard seeds, and that western mustards tend to use yellow only or yellow and brown seeds. Something like English mustard is notoriously hot for instance.

As a German, and I'm probably also speaking for Russians and Chinese here, I'm quite puzzled by this.

We consume more than one kind of mustard, one of them being aptly called "Süßer Senf" - sweet mustard[1], which is pretty far from your ordinary mustard in every way. It isn't hot at all, mostly sweet. You wouldn't use it instead of regular mustard (or vice-versa).

There's just no way "German mustard" is referring to anything specific enough to start making such comparisons. You'd have to narrow it down to regions of Germany to get anywhere, such as a Bavaria, the origin of sweet mustard.

[1]: This likely being the most popular brand https://en.wikipedia.org/wiki/H%C3%A4ndlmaier