I've checked this in my test server and, although, I'm not 100% sure if it's something I'm missing it should be reviewed, IMO.
In my test server I've the Spanish file: gradereport_grader.php properly installed and translated. It's contents are:
<?PHP // $Id: gradereport_grader.php,v 1.1 2007/06/23 12:04:38 barias Exp $
// gradereport_grader.php - created with Moodle 1.8 + (2007021502)
$string['grader:manage'] = 'Gestionar informe del calificador';
$string['grader:view'] = 'Ver informe del calificador';
$string['modulename'] = 'Calificador';
But, when I go to the gradebook page I get all the report, export and import plugins in English!
Not sure if it's something related with the use of the 4th parameter of the get_string() function or so, but it seems that the Spanish translation is never raised and should be.