I'm not sure what you mean by "perfected" and "mastered."
I've been learning (Amercian) English for over 58 years; I was born to it. I'm not sure what "perfected" means, but "mastered" to me means you know how to speak and write quickly without a constant need for dictionaries or grammar books. However, some British English idioms and slang might give me pause, as might some British vocabulary. I'm sorry, but what they call "lorries" we call "trucks." I've heard the word "lorry" enough to know what it is, as well as "flat," "bonnet," and "boot," but I'm sure if I ever travelled there I'd be just another "
Ugly American." (Petrol? Oh, you mean gas(oline)!)
So, I think I have mastered English, as I am not running to a dictionary looking up how to spell every other word or searching grammar book for correct syntax. Now as for French, I think I know just enought to aggravate a Frenchman with my limited vocabulary, my limited knowledge of grammar, and my pronunciation. C'est la vie.
Actually, to truly master a language, I think you need to be able to understand the humor. I get the impression that you are bilingual, so let me tell you a joke:
Q: What do you call a person who speaks three languages?
A: Trilingual.
Q: What do you call a person who speaks two languages?
A: Bilingual.
Q: What do you call a person who speaks only one language?
A: An American!
So, what do you see as the difference between "perfected" and "Mastered?"