i figured it out. You have to open "cforms.php" in the Plugin folder after the line 1109:
"\t\t\t".'jQuery(".cf_date").datepicker({buttonImage: "js/calendar.gif", buttonImageOnly: true, dateFormat: "'.$dformat.'" } );'."\n".
you have to add:
"\t\t\t".'jQuery(".cf_date").datepicker("option", "monthNames", ['.str_replace('"',"'", stripslashes($cformsSettings['global']['cforms_dp_months'])).']);'."\n".
"\t\t\t".'jQuery(".cf_date").datepicker("option", "dayNames", ['.str_replace('"',"'", stripslashes($cformsSettings['global']['cforms_dp_days'])).']);'."\n".
"\t\t\t".'jQuery(".cf_date").datepicker("option", "dayNamesMin", ['.str_replace('"',"'", stripslashes($cformsSettings['global']['cforms_dp_days'])).']);'."\n".
"\t\t\t".'jQuery(".cf_date").datepicker("option", "firstDay", "1");'."\n".
"\t\t\t".'jQuery(".cf_date").datepicker("option", "nextText", "'.stripslashes($nav[3]).'");'."\n".
"\t\t\t".'jQuery(".cf_date").datepicker("option", "nextClose", "'.stripslashes($nav[4]).'");'."\n".
Kejith