My use case is when interviewing candidates I often ask them to rate themselves on a scale of 1-5 in the languages they know, and then ask them increasingly 'tricky' questions in each language to get a feel for how their "personal" scale aligns to their real knowledge. This works fine if we have an overlap of several languages, but in the case where I know nothing or very little of one of the languages they know I lose that data point.
I find it valuable to know what a "I am a 1 at X" vs "I am a 3 X" vs "I am a 5 at X" means to them, since I've found little correlation between how harshly someone rates themselves and their true ability. Sometimes self-rated 5s are really 5s by my book, sometimes self-rated 3s are really 5s by my book, and sometimes self-rated 5s are really 2s by my book. So I want to know how "my scale" translates to "their scale". If it was more formalized I'd go as far as to get a "confidence quotient" for a person as self-critical and self-confident people can be fantastic engineers or horrible engineers.
Does anyone else do this process when interviewing?