What shared knowledge should audiences of my fair lady have to better appreciate the deeper meaning of the musical?

I think that the audience should have a shared knowledge of the English language.

The story depicts a transformation of a woman who is taught to speak English "properly". This change from gutter snipe to respected lady began by learning how to speak in English properly.

The audience must know how to speak in English to know the changes the use of language has undergone through the play.