This is a link to an excellent post about multilingualism and language disorders. I address this issue in all my seminars – the importance of finding the right speech therapist, and of doing therapy in both both languages, but it is better said by Mijam Blumenthal, who is a speech therapist specializing in treating bilingual children.
on raising bilingual children 
Thank you very much! I read several articles about speechtherapy for multilingual children. We still need qualified support. Most of the work is done by parents and as parents we need to have the insight. I’m lucky, as I have the linguistic knowledge. But what about all those parents who don’t?
I had that problem with my son. I am hispanic and my first language is spanish, his first year I would speak both languages, and my husband only spoke to him in english (he doesnt know spanish). When he turned 1 1/2 -2 he was talking less than any other kid i knew that was his age. I decided to take him to speech therapy and I even thought he was autistic. When he got evaluated he would speak both languages but the therapist only spoke english so I thought it was not fair for him. Once I started talking to him only Spanish and my husband only English his speech went from 1-10 and now that he is 3 he even translates for his dad. I believe bilingual kids need to be evaluated by a bilingual therapist it would of saved me many tears…
I’m sorry to hear that you had problems with finding a speech therapist who was bilingual – are you in the US? It is always amazing to me to find how many SLPs do not have training in bilingual speech development – they should all at least know the basics! Even if it isn’t possible to find a speech therapist who can evaluate in both languages, finding one who is “bilingual-friendly” and aware of the difference between monolingual speech development and bilingual speech development can be a huge help as well. Glad to hear that your son’s speech has caught up – it’s important to remember that he needs you to be “all Spanish, all the time”!