If German keyboards won't work with this, it's just trading one problem for another. (We'd be changing it to something else in six years.)
I don't know how to program this, but maybe... if either `<primary>plus` or `<primary>equal` is set, the other is also accepted (are = and + on the same key on the German keyboard?). But if neither is set (the user explicitly set a different keybinding), then neither are accepted.