actually it is a very complicated topic, i don't think there will be language confusion.
Yuen wa,
re the cases of your relatives, sorry that i would say " hit or punish , blame" are not good way for discipline; if we always respond to our bb's needs, have very good relationship with our bb, what he needs, we should understand. just like the bb under 1 year, he cannot speak , but we understand whether he is hungry or hot, or sleep, because we always care him, will not ignore his needs. like us, breastfeeding is a good way for discipline, for all of our babies, they are not difficult babies, and actually we seldom see our babies having hot -temper longer than 5 mins; also, if the mother or father is dumb(cannot speak), so the bb also is dumb, does it mean bb will have hot -temper because they cannot express their needs or feeling??
also, i met twin boys before, his mother only used one language(chinese), but both boys cannot speak well until 4 years, her mother was worry about this as afraid they are dumb, finally the boys can speak very well like adult after 4.1/2 years.. i think the oral development of baby will vary a lot.