My kids all spoke a little later than most. They spoke gibberish I guess you would say. Some words, but a lot of made up words. What's interesting is, other kids in my extended family have done the same, and then around three it all came together, and by 4 they were talking completely typically. It seemed to click.
One of my children's speech, they considered to be delayed and they did early intervention. He had a lot of hearing issues. However, he made a lot of sounds and interacted a lot with people - it just wasn't 'coherent' is how I would describe it. I, however, could understand what he wanted - he used gestures (pointed) and made sounds - they just weren't clear.
It did come up at the pediatrician's office to have him evaluated. So if your daughter's pediatrician is not concerned (you didn't mention this in your question), maybe there is not enough reason for concern. If it has not come up and you are concerned, then you have to be her advocate.
Good suggestions below :)
*Playing by herself a lot is not that unusual at that age. Mine did. Parallel play is very common (playing alongside other children, but not with them). Staring off into space - not so much. Maybe video that and just show the pediatrician as Diane suggests. Doesn't mean it's anything serious though. I have a niece who did that a lot as a child, and she outgrow it by the time she hit puberty. Still worth showing - if you're at all concerned.