Amy Tan's Mother Tongue is highlighting the difference in the many forms of a language used, not just by different individuals, but also by the same person when speaking to a different audience. I believe she is validating the individual usage of a language on the basis of it's expression and intent, not merely on grammatical correctness or grasp of vocabulary. If language is expressing properly to the intended audience, it is not to be considered lesser even if it is what could be called unrefined.
The back-and-forth conversation in which Amy Tan is the intermediary between her mother and her mother's stockbroker illustrates a perfect translation from "mother tongue" to "corporate American worker" speak. Ms. Tan