Oh come now, you would say that you know C (even if you're unwilling to say you "know" it for the purposes of debate). You don't have to have a language memorized to "know" it, and you don't "know" it if all you can do is write Hello World.
The answer is somewhere in between. My personal metric is: do you have a reasonably accurate estimation of what you don't know. The better you know a language, the better you know what you don't know.
If you really knew what you were talking about, you'd express more doubt about whether it's actually a good measure of how well you know any subject :)
The answer is somewhere in between. My personal metric is: do you have a reasonably accurate estimation of what you don't know. The better you know a language, the better you know what you don't know.