Fix install translation

When installing without a FQDN the installer can't output a valid error message:

    Notice: Undefined index: servername_should_be_fqdn in /var/www/localhost/htdocs/Froxlor/install/install.php on line 660

The corresponding string can be found for example in lng/german.lng.php but not in the used install/lng/german.lng.php.

First commit adds the string to the installer translation file and - since there are no other references in the main application - purges the lines from the generic translation files. Second one adds a missing word to the german translation.