But does the question really elicit the information you're looking for? CandidateA may know that her JavaScript could be better, but the (real) weakness she may trot out is that she's really nervous as a public speaker. It's not that she is hiding her JavaScript weakness, just that she's bothered less by it. CandidateB tells you that his JavaScript could be better, but he doesn't tell you that he yells at coworkers when they disagree with him. You're not necessarily going to get the info you asked for... for good or ill.
Why not ask, instead, "In which languages do you feel most and least confident? Why do you think that is?" or "We each learn differently: books, classes, hands-on experimentation. How do you go about learning a new language?" In the latter case, for instance, there's no obvious right or wrong answer which the candidate is motivated to "guess"; it's simply a matter of learning how this person operates.