When my daughter talks to herself, she leaves any semblance of English behind. I'm sure that there are English words there, but I'll be damned if I can understand any of them. The only thin that you know from the tone is that she's fine, because she only babbles to herself when she's in a comfortable mood.
When Hers Truly babbles AND talks in an extra-high piping voice, that means that she is quoting books to herself. She is still fairly incomprehensible, but you can occasionally catch a few words and figure out what she is talking about. Following her can be tricky though, as she may follow up any sentence from any book with any other sentence from any other book. This includes books that she read up to six months ago.