Misa (Gt.) from BRIDEAR write very well constructed English on Twitter, and she said she has learned the language since she was a kid.
Yashiro (Mary's Blood support guitar and tons of other projects) wrote excellent English.
Yuki (Gt. D_Drive), I think she has excellent English as well, sometimes post in English on Twitter and Instagram.
Female members of Moth in Lilac (Lisa13 Gt.), Ayano (Vo.) and Haruuna (Gt.) also did quite well in English.
Kanon (Bass, Ms. RedTHEATER), she can read and answer in English decently. I found that out while watching their twitcast broadcast.
Suzuhana Yuko (Vo. Wagakki Band and Hanafugetsu) also has decent English.