C.A.
I would be careful with this choice. There are many different kinds of speech delays. For instance, if your daughter has Apraxia, I don't believe putting her in a Spanish immersion program would illicit the best results. For children with Apraxia, adding Occupational Therapy is an important piece because it is a motor planning issue.
Find out what kind of speech delay she has and make sure she is receiving all possible therapy.
As a child, I received speech therapy because I pronounced certain sounds incorrectly and I had an Auditory Perception issue. Putting me in a second language program would have been deadly. I could not differiente between the different sounds 'a' makes even in English. My father's first language is French, my grandmother was a French professor and I took two years of French in high school. Despite their attempts, I can not speak a word. I equally had a problem with Spanish. I had to take Spanish in college and for my teaching credential. I still cannot speak a word.
I have heard of the need to work different muscles in a person's mouths to help with Speech but you want to make sure that her frustrations will be low. If she is struggling to learn a second language she may miss important instruction for learning to read and write in English.
Good luck!