I concur with Sophy C. and Julie P.
They note important suggestions.
For me, when my son was that age, I went to our Regional Center, the Zero-To-Three program (which is 'free') and got him assessed... because he too is delayed in speech. And again, these free services are only available until the child is 3 years old.
They do a FULL developmental assessment on the child... my son in his case, was found to be advanced in every area, but only in speech was he delayed. However, he did not have 'apraxia' or 'dysphagia' or anything else formally "wrong" with him. His speech-delay, was/is like many boys... they are just slower to talk.
I had the assessment done and have speech-therapy for my son BECAUSE (1) as a parent I wanted to formally rule out any other developmental concerns (2) it is free (3) it is very helpful in teaching the Parent AND child actual methods for facilitating speech (4) as a Mom, I wanted to get educated about how to help my son in any way possible...
AND, my son very much enjoys his speech therapy and his Speech Therapist. That is key.
My son, at 2.75 years old now, has progressed vastly in his speech. He is ALSO however, bi-lingual.... and he is vastly improved in BOTH languages. So he had to learn both languages and is very successful at it.
Presently, my son will be done with his Speech Services because he will be 3 yrs. old soon. But I never regretted it and my son loves it... he knows it is to help him talk. He is SO proud of himself.... and he chats away now.
Fortunately, my son has no other developmental concerns. The grandson of my Daughter's Teacher however, who is also the same age as my son, has Speech Therapy as well. But for him... he has other developmental issues in conjunction with his speech delay... and it was ONLY through their assessments that this was found out. For this child, he goes to a preschool... which addresses his speech, and because it was "recommended" from the Speech Therapist, the tuition is "free" for them. So this is an added benefit... of a program like this.
But yes, a child will talk when they are ready.
My son's Speech Therapist, says that 80% of her clients are boys... because it is gender based. For the girls who are speech delayed, they have other developmental concerns, not only the speech delay.
BUT, each child is different... your daughter clearly is bright and has great cognition... so perhaps she is just not ready to talk.
I'm sure she will talk more when she is ready.... but forcing it never helps. Einstein didn't "talk" until he was 3 years old. Lots of creative geniuses were that way, Mozart as well.
Don't worry... your girl is only 18 months old. Just see.. or, you can get her assessed through the program I mentioned. Each State seems to have one. Or ask your Pediatrician.
Keep in mind, that even babbling and singing is considered "pre-talking." No matter how much you talk to her... this will not necessarily speed up HER talking. But, from our Speech Therapist, I learned how to help in my son's articulation and forming his mouth/throat muscles and sounds and how to observe him as they make noises... so that I can help my son.
Early Intervention IS helpful no matter what. My friend, has a son that was delayed in speech as well....she didn't get speech therapy for him and just thought he'd "talk" when he was ready... but he is now 5 years old, and his speech is still not on par with his age and he does not "articulate" properly when he talks and some kids will ask "why" he still talks like a "baby."
But sure, every child is different... some kids will just take off talking when they are ready AND will be proficient at it. So it's up to the Parent to decide...
My son now talks! In both languages, and has great progress.
All the best,
Susan