Hey, if this is missing from your language file, you may add this manually, if that fails you can still just edite the theme's footer.php. Find the line
<?php if( (!defined('MULTISITE') || MULTISITE==0) && osc_get_preference('footer_link', 'bender') !== '0') {
echo '<div>' . sprintf(__('This website is proudly using the <a title="Osclass web" href="%s">classifieds scripts</a> software <strong>Osclass</strong>'), 'http://osclass.org/') . '</div>';
}
?>
and just replace the sprintf() with your string and link;
I hope this helps.