Language is part of the culture and a very important part of our daily lives. So much so that the way we speak may be different from conventional norms. However, that shouldn’t make us inferior or deemed unable to learn.
Reading and writing are two different skills and if we want students to learn them, we need to accept them as they are. The way they speak is not wrong. We need to teach them the standards of what we do in X setting, such as work or school, versus Y setting, such as at home, with family and friends.
If we make it okay to speak the way they do, we make it possible for them to learn how to play, what the authors in the book refer to, the language game.
It’s just like when you enter the theatre to watch a play versus to hear a concert or when you attend a wedding versus a karaoke party. The rules change and students need to learn them in order to be successful in the different settings.