Living in England in a city with a great university and a very international population I was not surprised to find multilingualism in my kids school. Speaking two or three languages is nothing special and even kids with four languages can be found. There are 23 different language backgrounds amongst the pupils at the school
My own children speak three languages, French as mother tongue, German as father tongue and English finely as the local tongue. In the beginning as parents we were not so sure what would come out of that language mix up. How can a child be not confused by the different native sounds and by hearing the parents talking to each other in a foreign language. However, everything developed quite naturally. I spoke only German to our children and my wife only French. Between both children is a four year age difference. At the age of 3 both children started half a day Nursery School. This was the first challenge. However, as both were already used to English through family friends and playmates on the playground and at Children Centres it was not too difficult for them to adjust. We thought that the influence of the playmates at the nursery and later at the school would be equally strong on the ability to acquire a new language as was the parental impact. We never tried to speak English to them at home and all stories read to them were in the parent's tongues. Both children progressed at school very fast and soon the were taken as native English speakers. At this time all three languages were equally well used. The boy, now nearly nine, reads books in three languages, however he struggles with proper writing except English, where he scores high. Unfortunately no foreign languages are taught in his primary school. The girl, just turned five, seemingly needs a bit more time with English writing and reading at school but still is good at it. Perhaps the reason for that is that she talks mostly German with her brother.
It is not true, that children with multiple languages are slow acquiring language ability. Often they show generally a faster and more confident approach. And they do not mix the different languages. Of course there are exceptions but mostly it works fine.
As we have a lot of friends and acquaintances with linguistically mixed households we are able to observe the language development of their kids. Apparently it is easier to acquire multiple languages at the same time, than two.
If only two languages are spoken at home, one is normally stronger. This is the local tongue. The child understands everything spoken in the non-local language but prefers to answer in the "stronger" local language, as he perceives it as a more appropriate communication tool. If the parents are not aware of it, after a while the parent with the non-local language gives in and communicates mostly in the "stronger" tongue. Only later they realize, that the child is not really bilingual, but than its often too late to reverse the process.
But how develops a child its language capability if both parents speak the same language in a foreign environment?