The fact that your 15-month-old only has her front two teeth is not a serious concern and should not affect her speech development. Still, she is certainly on the late side of normal with regard to the eruption of her teeth. You should have her see a pediatric dentist if possible; if not, a general dentist should be consulted.
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.