There's really only one way I can think of for her to have learned what she learned, aside from a bad writer not knowing shit about her character.
Anarky--Lonnie Machin--developed a teaching machine. It also allowed him to bridge both hemispheres, which in comic book science land, made him even smarter. If Cass somehow got access to that, or if Deathstroke did, it's conceivable for her to learn full english, how to read, how to write, ASL, Navajo, you name it. How long it would last . . . who knows.