Charlotte is Alma's mother, who has been devastated by her husband's untimely death and who remains severely depressed. In the novel, she is the most potent expression of the twisted connections between the four L's (again, those are loneliness, literature, language, and love).
Having lost the love of her life, Charlotte struggles to express her emotions. Okay, so we'll give her that one. Although it's argued that her husband would have wanted her to remarry after his death, to find that kind of happiness again, she's not even up for considering suitors, letting alone going out with them. Even her relationship with her children has suffered. Alma finds her mother's occasional attempts to connect with her oppressive and smothering. And the reader sees very little interaction between Charlotte and Bird.
Charlotte works as a book translator—and here's where those four L's come in, all wrapped up in a big messy ball. First, let's cover literature and loneliness: Alma describes her surrounded by a wall of dictionaries, literally cutting herself off from the rest of the world. From behind that wall, Alma hears her constantly exclaiming about how all her favorite authors should get "Posthumous Nobels" (2.28) (that is, a Nobel Prize given to someone after he or she has died), reinforcing the impression that she feels more connected to the dead than to the living.
Charlotte is originally from England. She learns Hebrew in order to speak with her beau, David, and learns Spanish in order to read The History of Love after he gives it to her as a gift. So, for her, language and love simply cannot be separated. Her affinity for languages is first paralleled by her great capacity for love. But by virtue of her needing to "learn a new language" in order to participate in her love affair with David, Krauss suggests that perhaps she is naturally more comfortable left alone with literature than communicating with others.