^^;; I thought I read Mana wrote Beast of Blood, since I have the interview.XD
Mana wrote the music, not the lyrics.
Kozi doesn't speak English, don't let lyrics fool you. He speaks just enough to understand what you might be getting at if you're clear and to the point. And he can reply with simple phrases here and there in a very good accent (which is odd) but that's all. He kind of panicks too (or at least used to) when people try to talk to him because he wants to communicate but can't. Which is why he appears shy to fans who meet him.
I have no idea what Mana's knowledge of English is and he always goes on about speaking French and we know how much of that he speaks so... lol. His lyrics are too fragmentary and incorrect as it is for me to take them as an indication.
Klaha seems to have a better knowledge than the rest of them because he feels at ease to write all the time, He might have improved since POM days Flowers... It's been a while. Not saying that he's perfect or anything but I think he knows the language to some degree.
Yu~ki I have no clue.
But I'm going to vote Gackt because lyrics aside I've heard Gackt speak English - as in, have a proper conversation in it - and honestly, pronounciation and japanisms aside he was fine. I'm possitive he can speak it to a very working level.