It is entirely possible to teach an infant two or even three languages, and four is not unheard of.
In Europe, a great many toddlers learn four languages with little or no difficulty. Some countries in Europe offer children in the early grades of elementary school the possibility to learn to read and write in their ‘home’ language
The main requirements for this learning are: the parents speak only their mother-tongue to the child because the child will have some reason to learn the languages (motivation); and there is reinforcement of some kind for these languages, preferably outside the home. If the language of the environment is a third language, then the child will easily learn the third language once they start playing with neighborhood children. I t is very possible a child learn 2 or more languages at the same time.
Parents who want their children to learn their mother-tongue must realize that it will take work, beyond simply speaking their mother-tongue all the time to the child. Especially if the spouse speaks another language, which is the language of the environment, the parent speaking the ‘minority’ language will have to be sure that there is sufficient input for the child to learn and reinforce what has been learned. This means things like reading out loud (this should go on until the child learns to read on their own, and for a few years afterwards until the child says stop), singing to them and teaching them songs and nursery rhymes, showing video films in the parent’s language (radio is not as good as there are no visual clues), and having other adults or children talk to the child in this language (grandparents are invaluable here). Taking the child to visit in a country where the parent’s language is the language of the environment is also a good idea
There is considerable debate among linguists as to when the ‘language learning window’ closes, if it closes at all. However, there does seem to be an ‘optimal’ age for language learning, when the child’s mind is still open and flexible, and not cluttered with all sorts of other learning, not to mention the society’s views on which languages are ‘prestige’ languages, and which ones are regarded by the society as of little or no importance. The latter affects motivation: children will be admired for speaking a ‘prestige’ language, and teased and bullied for speaking a ‘non-prestige’ language. When the mind is being taught many many other things than language, there is less ‘processing space’ left for language learning.
At the moment, the ‘optimal’ time for learning a second language appears to be ‘at the same time as the first language‘, i.e. in the home beginning at birth to three years (providing the parents speak these two languages as their mother tongue). The next best time for learning a second, third, and even a fourth language, appears to be between the ages of two to seven years. A third period for learning a second language is from about ten to thirteen years of age, this is in cases when the second language is not the language of either the parents or the environment. This is the reason behind the push to introduce ‘foreign’ language learning into the curriculum of elementary schools, in the grade when the child is about ten-eleven years old.
Bilingual kids are smart! 