I believed that her English reflected the quality of what she had to say. That is, because she expressed them imperfectly her thoughts were imperfect. How does this detail develop the central idea Mother Tongue

To be reticent is to not reveal one's thoughts readily. In this case, it is the reticence of the author. The author does not reveal their thoughts readily. If the narrator or author refuses to do something, it is made known that he is doing something. The something here is him refusing. If he acts as if the situation is normal, I'd assume he is acting in such a way that would be nonreactionary of the actions from the provoker. I would safely choose the last option as sorrentino's exercising of authorial reticence.