Is there a number of languages which is too many for a baby to learn?

In principle, no. However, remember that learning a language does depend on hearing the language, and hearing a wide variety of words and grammatical constructions. The more languages a baby hears, the less time they will be hearing any one language, and so the less likely they are to become fully proficient in the language. One likely scenario in a family where there are, say, four languages (perhaps one both parents share, another from the mother, another from the father, and another in the community), is that one or two of those languages will become dominant (such as the shared family language and the community language) and the child will become fluent in those; they may not learn the other two languages to the same level, but they could still play an important cultural and emotional role in the family. Remember that there are many parts of the world where using 3, 4 or 5 languages in daily life is the norm! Read more about this and the advantages of speaking more than one language here.

We would like to raise our baby as bilingual, but only one of us is bilingual. Should the other caregiver learn the language too?

It’s always great to have a go at language learning. In this case, for the monolingual parent to learn the language could also mean that they feel more included in family relationships, which could have a positive effect. However, it’s not essential, and many families do follow a strategy where each parent speaks one language to their children (and may not understand the other language). You can read more about this strategy, and the ways that children creatively use all their languages here.

Is there a particular age when it becomes harder to learn another language?

This is a much debated question! Overall, the evidence suggests that, for many possible reasons, it does become harder to learn a language to the same level as someone who grew up speaking that language as you get older (into adulthood). This may be because of changes in the brain, changes in environment and motivation, changes in the way we learn languages, or all of these. Young children, however, are able to learn second (or third) languages well, even if they haven’t heard them from birth. And in general, the message is: it’s never too late to start! (But you might need to adjust your expectations). Read more here.

What advice is there for families with children with a developmental disorder (e.g. autism, ADHD, Down’s syndrome?

Research to date suggests that for many developmental disorders, there is no evidence for a negative effect of continuing to speak more than one language, and there may even be a positive effect. The Royal College of Speech and Language Therapists now advises keeping up both languages in most circumstances. Imagine a family where everyone speaks, say, Spanish at home, but because one child is autistic, the caregivers now only speak English to her: this actually deprives the child of opportunities to learn and enjoy social and language skills, and might mean she is excluded from many family conversation. You can find a review of the evidence here:

Uljarević, M., Katsos, N., Hudry, K., & Gibson, J.L. (2016). Multilingualism and neurodevelopmental disorders – an overview of recent research and discussion of clinical implications. Journal of Child Psychology and Psychiatry, 57(11): 1205-121

And some newspaper-style articles from the same research group here and here:

What about sign languages?

A sign language is a language – a full linguistic system, which happens to be in a different modality. This means that children learning sign language can be multilingual – if they are exposed to more than one sign language (for instance, British Sign Language and American Sign Language are actually completely different, or they might learn BSL and Turkish sign language) – and/or multilingual and multimodal, if they learn a sign language and a spoken language (for instance, if they have deaf parents but are hearing themselves).