In this context, you can have mastered something without being outstanding at it. I've mastered the art of juggling three balls in the air but although I can keep them going, it's not a thing of beauty and my mastery of juggling is pretty mediocre.I don't see that mastery comes in various grades, e.g. outstanding, mediocre and poor.
Well it doesn't sound brilliant, AFG. It doesn't sound to me any better than (e.g.) good.It sounds crap to you, really ?
So if I write on my resume that I am "Proficient in both spoken and written English", it doesn't mean much?
I am pretty sure this falls under Muphry's Law."Excellend command of both written and spoken English".
(I think you meant "Murphy's Law". Murphy strikes again. )I think fluency caps out at fluent. You could add qualifiers, as in "Fluent in English; published three articles in a peer-reviewed English-language journal" to prove that you are capable of communicating fluently at the highest level. Tacking on superlatives sounds insecure to me though.
I am pretty sure this falls under Muphry's Law.