MDL-10490 Adding a couple of language strings

lang/en_utf8/grades.php

@@ -127,6 +127,8 @@ $string['excluded'] = 'Excluded';
 $string['excludedhelp'] = 'If -excluded- is switched on, this grade will be excluded from any aggregation performed by any parent grade item or category.';
 $string['expand'] = 'Expand Category';
 $string['export'] = 'Export';
+$string['exportletters'] = 'Export letters';
+$string['exportlettershelp'] = 'For grade items that use grade letters instead of values, the letters will be exported instead of the actual values.';
 $string['exportplugins'] = 'Export plugins';
 $string['extracredit'] = 'Extra Credit';
 $string['extracreditwarning'] = 'Note: Setting all items for a category to extra credit will effectively remove them from the grade calculation. Since there will be no point total';