Virginia - I think what you say is true in a society where the predominant language is English and that is what the child will learn to speak naturally.
However, Lois is in a society where English is not the predominant language - in fact, it's spoken only by the few people she's close to. English is still her 'own language' but it's not the language Diddi is going to learn at school. Therefore, if she taught him to read and write, I think it's perfectly reasonable that she would think of it as 'teaching him English'.
Corrina.
