Hi Matthias,
That br issue was an unfortunate last minute bug. If you download 2.3.1.7 again and replace just the mojoPortal.Web.dll it will fix it.
The editor should use whatever the default language of the site is. To force it to German, add this to your user.config then touch Web.config
<add key="UseCultureOverride" value="true" />
<add key="site1culture" value="de-DE" />
assuming your site id is 1.
Best,
Joe