Hi,
First, note that Mondial Relay is not available in Germany as far as I know.
There is a check in the Mondial Relay plugin to only display the shipping method for the countries where Mondial relay is available:
if(!in_array($country, array('FR','BE','LU','FX','AT','ES','IT','NL','PT')))
return false;
As you can see, DE (for Germany) is not in the list.
Now, IT and PT (for Italy and Portugal) are in the list. So maybe there is something else going on.
If you restricted the Mondial Relay shipping method with its "zone" setting, I would recommend not doing that. The plugin already has a check for that so it's not necessary, and, if the zones are not configured properly, it's possible that it is the cause of your problem.