The topic of bilingualism never grows old; I am constantly asked about it by a range of people such as moms at mother's groups, babysitters, teachers, friends, and even other SLPs.
Mostly, they want to know if:
1) Bilingualism causes language delays
2) There are any benefits to learning a second language at a young age
3) There are any real benefits to learning a second language, period.
Bilingual language acquisition and development is still a very new and, therefore, exciting field compared to its monolingual counterpart. There are still lots of grey areas around defining what bilingualism means and who can qualify, at times muddying research methodologies.
Regardless, the general consensus is that:
1) Bilingualism does NOT cause language delays;
2) As with other things, it is best to pick up a second language at a younger age when the brain is still at its most absorbant stage;
3) There are benefits to learning a second language. For example, it helps to create better meta-linguistic awareness (i.e., more awareness of sounds, and language rules) which in turn could help with reading and spelling, or even picking up a third language eventually.
What about a child with a diagnosed language delay? Should he/she still continue learning a different language?
My personal recommendation would be: if the child comes from a family that speaks a language different from the majority of the population (such is seen in migrant families to English speaking countries like Australia or the US), she should absolutely continue speaking her home language while also learning the common language of the country.
There are cultural implications when a child stops speaking their home language. Also, a child with a language delay or disorder will need explicit teaching of rules and words, regardless of the language(s) they acquire. So it really does not seem necessary to stop the child from learning one of the languages.
If the child with a language delay or disorder is being asked to take a second language at high school to fulfill academic requirements, and has little to no interest in actually participating (which a lot of kids do anyway, unfortunately!!), then we might have to reassess the situation and recommend the child does something else that is more fulfilling and potentially useful. This is just my personal opinion - if people have any other thoughts, please feel to share them.
There is a great post by Dr. Brian Goldstein, a prolific researcher in the field of bilingual speech and language pathology and the Dean of an SLP faculty in the United States, called "Providing Clinical Services to Children: Stop doing that!" that was published on the blog of another really well known researcher in the (multi) linguistics field who is herself multilingual Dr. Madalena Cruz-Ferreira. I encourage everyone to read it, both clinicians and parents, as it busts myths about bilingualism and language delays in clinical practice with citations of the latest research.
While I'm obviously a proponent of bi(or multi)lingualism and am convinced it does not cause language delays or disorders and should continued to be taught to children regardless, I wonder if the same can be said of bilingual reading: should a child with a reading difficulty (dyslexia) in one script continue learning to read in another? That's an area that I haven't seen much research on at the moment and would welcome any guidance to relevant literature!
Meanwhile, next time someone asks you if bilingualism is the reason for their child's speech and language delays, please answer with a confident "No!"