I think what we're missing here is: Exactly what are we talking about? Teachers? Actors? Translators?
Speaking from personal experience, I have seen some pretty awful English produced by Germans who
think they know English well enough not to require a proof-reader. They may be able to reel off an impressive list of parts of speech, but only a few produce clear and idiomatic English.
That said, I would definitely recommend that any native English speaker hoping to teach
ESL learn another language.