I helped with interviews at a company a few years ago. We eventually stopped asking candidates to rate themselves on a scale of 1 to 10 in the language we used.

The answer was consistently 7 or 8 (with one 6 if I recall correctly), independent of experience, skill, or understanding. Since we were doing technical interviews and testing, we could actually judge the person's skills for ourselves, so we dropped the question as a waste of time.