The author of the language is not necessarily the most knowledgeable in how to use it.
Maybe not the best individual but still very likely someone who deserves such a high rating
I think it helps to say "relative proficiency in each language". For example, I would say I have 2/5 in Python, and 5/5 in PHP. I'm not saying that I'm the best PHP developer in the world, just that personally I'm much stronger in PHP than in Python.
But relative to what?
How do you compare two candidates with "5/5 in PHP"?
I want to know your absolute proficiency.
If you have 5/5 for a language, I expect you to be as knowledgeable as the authors of the language