I grew up speaking two languages – my mother tongue was English and my second language was Afrikaans. My mother knew Xhosa and German and my father knew Irish so I learned bits of those as well. I don’t even remember learning any of these languages. It just happened.
I married an Irish man who had two children. He had been living in Stockholm for about twenty years at that stage and had made a conscious decision to never speak English to his children. He said he wanted them to learn one language well, rather than two badly. This was, I believe, the prevailing thought of the time. As they got older they often said they wished they had learned English when they were small. They felt it would have given them an advantage. When I became a granny, I was given the task of speaking only English to my grandson. So that he would learn as he grew older.
Most children can learn several languages when they are young, and can keep them separate. There are. however, exceptions. Some children do battle in a bilingual environment when they are small. My guess would be to play it by ear.
In the past, researchers have thought that children learn the differences in languages by noticing how often a word is used in a language. By doing that they identify a word’s importance in a language and notice the differences between those words in different languages.
New French and Canadian research shows however, that it is the melody and speed of the pronunciation of a word which allows them to hear the differences. These researchers let children of seven months’ of age listen to a very simple language with a grammar and melody which was very similar to the children’s two mother tongues. They could see that the children could see the differences between languages, even though they were so similar. By doing this they could strengthen the theory that small children learn to handle two different languages at an early age just as well as they handle one.