The moving vignette you are referring to in this excellent collection of stories based in a Latino barrio is entitled "No Speak English." It concerns primarily the mother of one of Esperanza's neighbours who came to live with him with a baby boy, that we can infer is her grandson. Mamacita keeps all to herself because she doesn't know how to speak English and this isolates her, as does her own daydreaming about life back "home" and the severe homesickness she suffers whilst listening to Spanish radio songs. What makes it worse is that the "baby boy" she looks after is coming to treat the United States as home, which is another blow to her identity as she tries to define herself by her Latino heritage:
And then to break her heart forever, the baby boy, who has begun to talk, starts to sing the Pepsi commercial he heard on T.V.
Of course, the Pepsi commercial is such a visible symbol of American life, indicating that he is not going to be growing up singing the same Spanish songs and yearning for her "home." His home will be here, in the United States, and her desperate attempt to make him only speak Spanish is as doomed as her dream of returning to her "home."