I think, "knowing a programming language" is to have a basic understanding of that language, and their syntax, and what compiler to use to run that specific program in that language. That can be sum up saying that somebody knows a programming language.
However, there would be the follow-up questions on his understanding level of that language:
1) How depth your knowledge of that specific programming language?
2) what is your experience in that programming language? What are the applications you have developed by using that programming language?
That would gauge somebody's understanding level of a programming language.
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