The chances are that she will gradually relinquish her private language and switch to the language used by the rest of the family. In the meantime, interact verbally with her as much as possible. Although you have no clue what she is saying, nouns may provide a bridge between the two languages. Hold up some object she likes (a spoon, a favorite toy, etc.) and ask, “What is this?” If she gives you an approximation of a word, say distinctly but not harshly, “I call that a _________.” If she said her word clearly enough for you to repeat it, say something like, “You call that an _______ but I call it an ________. I said it your way, now you say it my way.” If she doesn’t comply, don’t make an issue of it. Just use the procedure frequently. As she gets a little older, try it with verbs.
Although it is not common, some children seem to talk in sentences (or paragraphs) before they use individual words correctly. They’ll run on and on, and maybe you’ll be able to make out one word. Then a month or so later they will do the same thing, but then you can make out two words. And so on until the entire sentence comes out in recognizable words. (I have a nephew who did this, and he is now a distinguished attorney with impeccable language habits.)
For now, the most important thing is that you not get your granddaughter anxious about talking. Always respond to what she tries to say as honestly as you can: “I’m not sure what you want. Can you help me by pointing? Or let me know if I misinterpreted what you mean.” And make sure you read to her every day, allowing her to point to pictures when you say different words. If she uses her private word, simply say, “We call that a ______.” Nothing helps language development mature better than consistent and pleasurable reading.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.