I have seen a number of children just like the child you describe. They can talk up a storm, but they have trouble responding to what other people are communicating. This child might say, "I want to go outside," and you say, "Oh! That sounds like fun. What do you want to do outside?" Instead of responding to your question, the child might start to talk about a toy she has in her hand.
It's not always easy to see that this behavior can be a sign of a language problem. It would be easier to recognize a problem if the child were not verbal at all, or significantly delayed in expressive language. It's a more difficult challenge when the child is like the girl you describe, who is very verbal but marches to her own drummer.
Fortunately, you are on your way to identifying the problem. The next step is to decide what to do. There are things you can do right in the classroom.
Stir Up Some Conversation
Exercise the child's receptive language. Try to build up to having long conversations. Talk about everyday events. You might just listen to the child and repeat something she's saying in a summary way to encourage her to go on. You want to challenge the child, but not too much at first. Start by raising questions or making comments that she has to understand in order to go on with the conversation. Center your conversations on things the child is interested in, so she is motivated to try to comprehend what you're saying.
To get the child thinking, ask multiple-choice questions. "Do you like to go outside because you want to play, because you're tired, or because you want to go to sleep?" Two are silly choices, and one is a good choice. Always give the good choice first and the silly choices second, so the child won't just repeat the last diing that she hears.
Invent Talking Games
Play "following directions" games. For example, if the child wants something, say, "Oh, let me help you get that. But first you have to pull me up from my chair. Then you have to show me where it is." Keep making demands-things the child needs to do for you first-starting with only one thing, then two things, and so on. In this way, the child has to process your requests in order to get something she really wants.
Play "treasure hunt." Hide something the child likes and give her clues as to where to find it. Begin with simple clues, such as, "What's in the green box right behind you?" Gradually offer more complicated clues, like, "It'll be in the red box on the stairs, near the radiator." It's also helpful to integrate visual clues with verbal clues to help receptive language along. In your treasure-hunt games, you can show the child pictures of where something is hidden as well as tell her where it's hidden. Eventually, see if the child can find it by relying on the verbal clues alone.
Practice like this is very useful if the child simply has some unevenness in the way different parts of her nervous system are developing. Her expressive functions may be developing faster than her receptive ones.
Arrange for Evaluations
When there are receptive-language problems, it's always a good idea to make sure the child is hearing properly. Consider an audiological evaluation, even if the child is obviously hearing some things. Some children hear loud sounds at certain frequencies, but not at others. Other children may be oversensitive to certain sound frequencies. A high-pitched voice or the noise in a busy preschool may make it hard for these children to listen properly, even though they are able to hear and process sounds in a one-on-one situation. There are a number of other medical problems that can contribute to receptive-language delays, so it's wise to recommend that the child have a general pediatrie evaluation as well.
If the child is not making adequate progress after you help her to exercise her receptive skills, bring in a speech pathologist to do a formal assessment.