There is no current research to suggest that speaking a language other than English will harm a child (though many people think this).  There are numerous articles that support that use of the home language.  Please read the following research:

• Bird, E. K., Cleave, P., Trudeau, N., Thordardottir, E., Sutton, A., & Thorpe, A. (2005). The language abilities of bilingual children with down syndrome. American Journal of Speech-Language Pathology, 14, 187-199. 

• Genesee, F., Paradis, J., & Crago, M. B. (2011). Dual language development and disorders: A handbook on bilingualism and second language learning- Second Edition.  Baltimore: Paul H. Brookes Publishing Co. 

• Kremer-Sadlik, T. (2005). To be or not to be bilingual: Autistic children from multilingual families. Paper presented at the 2003 at the ISB4: Proceedings of the 4th International Symposium on Bilingualism. Retrieved October 20, 2011 fromwww.cascadilla.com/isb4.html  ( Also, check out SFARI below)

• Perozzi, J. A., & Sanchez, M. L. C. (1992). The effect of instruction in L1 on receptive acquisition of L2 for bilingual children with language delay. Language, Speech, and Hearing Services in Schools, 23, 348-352.

The Simons Autism Foundation for Research Initiative (SFARI) reports on two 2011 studies showing no negative effects of bilingualism on children with autism.
http://sfari.org/news-and-opinion/in-brief/2011/cognition-and-behavior-bilingualism-doesnt-hinder-language-in-autism 

Cognition and behavior: Bilingualism doesn't hinder language

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By Virginia Hughes
8 November 2011

Mother tongues: Bilingual parents should not be discouraged from exposing children with autism to multiple languages, two studies suggest. Growing up bilingual doesn’t impair language skills in children with autism, according to two studies in the Journal of Autism and Developmental Disorders.

For the first half of the 20^th century, many scientists believed that learning more than one language impairs a child's cognitive development. That idea has been debunked in the past few decades, but for children with autism, some clinicians still recommend learning only one language.

The two new studies, the first to rigorously investigate this issue, challenge that idea. The first, published 22 September, studied 75 children with autism, between 3 and 6 years of age, separated into three groups: 30 who were monolingual, 24 who were exposed to a second language before 12 months of age, and 21 who were exposed to a second language after 12 months of age¹.

After giving the children seven questionnaires and diagnostic tests, including the Autism Diagnostic Interview-Revised and the Social Responsiveness Scale, the researchers found no significant differences in language ability among any of the groups.

The second report, published 27 September, studied 28 children diagnosed with an autism spectrum disorder, also between 3 and 6 years of age. Half of the children spoke Chinese at home and English outside the home, and the other half spoke only English².

The researchers found no differences between the two groups on four different language and vocabulary tests. What's more, after controlling for non-verbal intelligence quotient scores and age, the team found that bilingual children have larger English vocabularies than monolingual children do.

Some studies on typically developing bilingual children show that their vocabularies in both languages are just as large as or larger than those of children who learn one language³. Others report that bilingual children tend to have smaller vocabularies⁴.

Researchers from both new studies conclude that bilingualism is not harmful to children with autism, and that clinicians should not advise bilingual parents to teach their children with autism only one language.

References:

1: Hambly C. and E. Fombonne J. Autism Dev. Disord. Epub ahead of print (2011) PubMed

2: Petersen J.M. et al. J. Autism Dev. Disord. Epub ahead of print (2011) PubMed

3: Pearson B.Z. et al. Lang. Learn. 43, 93–120 (1993) Abstract

4: Bialystok E. et al. J. Cogn. Dev. 11, 485-508 (2010) PubMed

Here are two (of may highly-recommended) videos that Brenda K. Gorman, Ph.D., CCC-SLP put together.  They are relatively short and simple for non-specialists, which many people find appealing.  References are listed on the Lingua Health website.
 

Myths About Bilingual Children: http://youtu.be/LVYhpCprtzQ

Can Special Needs Kids be Bilingual?: http://youtu.be/vOhWg0YeIMs

Addressing Clinician–Client Mismatch:
A Preliminary Intervention Study With a Bilingual Vietnamese–English Preschooler

Giang Phama
Kathryn Kohnerta
Deanine Mannb
a University of Minnesota, Minneapolis
b Centennial Early Childhood Special Education, Lino Lakes, MN

Correspondence to Giang Pham: tangx098@umn.edu

Purpose: This project examined receptive vocabulary treatment outcomes in the two languages of a bilingual preschooler with moderate to severe language impairment.

Method: A series of single-subject experimental designs was used to compare English-only (EO) and bilingual (BI) approaches to receptive vocabulary treatment. The participant, Nam, was a boy age 3;11 (years;months) who was learning Vietnamese as a first language at home and English in his early childhood education program. Treatment was implemented by an EO interventionist using a computer interface and prerecorded audio files in Vietnamese and English. The dependent measure was the percentage of items that were correctly identified in each language.

Results: Combined studies revealed that the BI approach increased Nam's attention to task and was as effective as the EO approach for increasing his receptive vocabulary in English. Nam made vocabulary gains in both treatment conditions; receptive vocabulary gains were evident in both Vietnamese and English.

Conclusion: This project showed that it is feasible for an EO clinician to promote gains in both the home and school languages of a BI child through creative collaborations with BI colleagues and the use of technology. Replication with additional participants and treatment activities is needed to make further generalizations.