I'm French and I speak english everyday since 5 years (foreign girlfriends) I just can't pronounce this language correctly, the mouth positions required are simply to far from my native tongue. And in the morning it's even worse. I think there is an elocution max level for each of us that's very hard to pass (I suppose that would involve some kind of specific elocution training), whereas the vocabulary always grow. I've met people living in the same foreign country for 20 years and still have a very strong native accent.