Why does Annie think about language?
Annie thinks language is essential for Helen to connect to the outside world; this is demonstrated when she says that language is to the mind what light is to the eyes.
Perhaps because it is the essential way of communicating, and something she must teach Helen.
She felt that by communicating through language, it is a best portal for Helen to relate to the outside world and provide a communication tool for her to know others better and this would broaden up her social circle. She felt that "First, last, and—in between, language.... Language is to the mind more than light is to the eye." as she was once blind before and she knows how the power of language had benefitted her entire life tremendously.