Hi Stefan,
I'm really not sure how the display language could really affect whether the system icon button is hit tested properly. Do you see the same issue on a regular Window that has our Themes' WindowChrome applied? Note that WindowChrome uses the same kind of Window theming that RibbonWindow does, but RibbonWindow has a slightly altered template from WindowChrome.
I tried reproing this in Windows 7 US but couldn't. Can you repro it in Windows 7 US when you have a German display language active?