You need to test in both English and French (for example, basically one lang where they say 0.5 and on where they say 0,5). You need to test both manual grading using the 'Make comment or override grade' link when reviewing a quiz attempt, and manual grading using Quiz -> Results -> Manual grading. You need to test Valid grades are accepted. Leaving the box blank does not set a grade. (If it was buggy, it might set the grade to 0.) That you get a validation message if the number you type is out of range too high or too low.