Fair enough, I'll give you your 1/3.

Not sure about your last comment though. I've never met a second generation immigrant whose English has suffered as a result of their parents'. My girlfriend's father is Dutch, and her English is perfect. So is his, I might add. And I've met a lot of Chinese Australians/Canadians in Japan who speak English 100% naturally. Some 2nd generation Australians have slight accents though, notibly the Greek community, but the only mistakes they'd make in their speech would be natural mistakes that all speakers make.