Yes, as long as these languages are in the family or in the children’s environment and will therefore feel natural and useful to the child. A multilingual city, such as Brussels is therefore a favorable environment to bring up a multilingual child.
How many languages can children learn? As many as they need and are motivated to learn. There is no limit to the number of languages other than the amount of regular exposure/interaction that children can actually have in the different languages. Children will learn languages (as everything else in life) if they sense a need for learning them in their everyday life. If people around them (family, caretakers, school, friends, etc.) speak different languages to the children consistently and sufficiently, they will learn them.
Firstly, it is crucial that parents should express a positive attitude towards multilingualism, so that the children will feel that knowing more than one language is important and appreciated. Secondly, parents need to decide on a linguistic strategy well adapted to the family situation and its surroundings and to stick to it consistently.
If the parents have different native languages and these languages are standardized written languages, there is little doubt that it is best for them to adopt the so-called OPOL strategy (One parent one language): the parents each consistently use their own native language with the child, and they should do so as early as possible, ideally from birth. This has the advantage of creating a more intimate link with the child and with their cultural roots. It is also likely to give a good linguistic model because the parents speak the language they master best.
If the parents do have the same native language, it is still possible to produce bilingualism at home through using the so-called artificial strategy: one of the parents uses with the child a second language that is not his or her own mother tongue. This requires a very good mastery of the second language and a lot of determination, since this is not the most natural choice. In most families with a single native language, however, the children will become multilingual mainly thanks to attending a school that operates in a language distinct from the home language, either partially (see below: What type of school is best for making our children multilingual?) or entirely (see below: Is it a good idea to send children to school in a language different from the home language?).
In each of these basic strategies, the role of parents and school can often be usefully supplemented by other people in frequent contact with the child: grandparents or other relatives, neighbours, child minders, teachers, youth movements, etc. These people can help strengthen the linguistic competence children owe to their parents or their school, or teach a different language to the children and enable them to practice it on a regular basis. For instance a child born in a homogeneous French-speaking family could learn Dutch by having a Dutch speaking child minder and later joining a youth movement or sports club that operates in Dutch.
The school language is likely to become the strongest language of the child, so this is an important consideration in school choice, along with others — such as the school’s distance from home or its “educational project” — and what type of school is best is highly dependent on the family’s specific situation.
In the case of a bilingual family, for example, if a choice is possible between the two languages as regards the medium of instruction at school, it is better, other things equal, to choose the language less present in the home environment. If the possibility exists, the parents may also choose to send their children to a school that operates in a third language, so that their the children can grow up trilingual — even quadrilingual if the school offers immersion in a further language other than the home languages. For this to work best, however, it is important that one parent at least should know or acquire the school languages in order to follow the children’s school work, while sticking to the home languages for most other purposes, and make sure there is adequate exposure to each of the languages involved.
In the case of monolingual families, the most obvious choice, if it exists is an immersion school with the home language as one of the languages of instruction. (See What is the best method for teaching a language?) However, for most families in Brussels, this is not a real choice because their home language is not a medium of instruction in any Brussels school, or only in schools that charge fees they cannot afford, or only in European Schools to which they have no access. Even for families with French as the home language, this choice often does not exist. Access to French-language schools offering immersion teaching in Brussels is limited (10 primary schools and 18 secondary schools) and complicated. The main obstacle is the overall shortage of school places, coupled with a registration procedure that takes various criteria into account (such as socio-economic background or distance from the school). Moreover, educational continuity between primary and secondary school is not assured in immersion teaching.
It would certainly be good if more schools adopted immersion schooling, or if genuinely bilingual or multilingual schools could be funded by the Belgian authorities (see Are there bilingual schools in Brussels?). In the meanwhile, many parents send their children to a school operating in a language different from their own. Indeed, many have no other choice. (See below: Is it a good idea to send children to school in a language different from the home language?)
Submersion education refers to a situation in which pupils attend school in a language different from their native language without the latter being used to any extent as the medium of instruction. Submersion education, so defined, was common practice throughout Europe when the official language of the incipient nations was the only medium of instruction and differed often deeply from the pupils’ home dialects. It is common practice in today’s Africa, where the colonial language often serves as the only medium of instruction for children with African native languages. And most children from immigrant families in Brussels today have no other option. There is simply no school that offers teaching in their native language, and no hope whatever, for many of them, that this will ever happen: even if standard Arabic were adopted as a language of instruction in some schools, it would still be significantly different from the Moroccan dialect darija, and extremely different from the Berber dialects.
Attending school in a language that one did not know at all before entering it can be a traumatic experience. As a result of educational submersion, children can suffer a lasting linguistic handicap that affects their learning of all subjects and sometimes leads to permanent cognitive and psychological damage. The frequency of these effects has given submersion education a bad name among experts, especially as it is often associated with the stigmatization of the children’s native languages.
Nonetheless, it can succeed. Many pupils manage remarkably well and end up mastering the school language just as well as its average native speakers, while remaining proficient in their home language. Whether or not submersion education succeeds, depends crucially on many factors, such as the teachers having the attitude, the skills and the time to take account of the special needs of children with little or no prior knowledge of the school language, the linguistic composition of the class groups, the ability of parents and teachers to communicate with one another, the consistent use of languages at home and the extent of exposure to the school language in the media or in extra-curricular activities and at the pre-school stage.
In the Brussels context, identifying and boosting these factors of successful submersion is of primary importance. It will be essential to learn as much as one can from the achievements and difficulties of the many Dutch- and French-language schools that have no option but to offer submersion education to many of their pupils — and to keep experimenting in order to address more effectively this crucial challenge. 
Children are genetically equipped to learn several languages as they grow up. However, if young children are not to end up confused, consistency is important. Ideally, each of the parents and other adults in regular contact with a young child should endeavor to always address that child in the same language. They should avoid shifting from one language to another in the same sentence, the same conversation or from one day to the next. In particular, they should not try to speak with the young child the school or pre-school language when they themselves do not master it very well. This does not help the child learn the school or pre-school language. On the contrary, it makes it more difficult for children to work out the pronunciation and grammar rules that underlie the conversations they are exposed to. Sticking to their own native languages is far better.
This ideal, however, is often unachievable, particularly when the native language of the parents is an unwritten dialect without an established code and perforated by borrowings from other languages. What the best strategy is in this case will depend on the specific circumstances. Above all, parents in that situation should not feel guilty nor let linguistic difficulties spoil their emotional relationship with their children. They should still take pride in their native languages and, when realistic, try to transmit them to their children, while early and regular attendance of a kindergarten in either French or Dutch must help their child secure the firm acquisition of one standard language before school starts.
In any event, the concern for consistency can be relaxed when the children are in their teens and their various languages are firmly established and distinguished. Moreover, it does not prevent parents from speaking another language, including with each other, in the presence of the child. Nor does it prevent them from making exceptions to the basic rules, clearly demarcated in terms of activity, time and place. The child will understand that there is a temporary change of rules and will not be confused. This could be done for example when discussing schoolwork, reading a book or during an outing or holiday.
Some mixing of languages is a normal stage in the linguistic development of a bi- or multilingual child and therefore nothing to worry about. Many parents report such a stage. As the child grows, the involuntary mixing tends to decrease. However, some conscious forms of mixing can become permanent features of communication among multilingual children and adults. One such form is “code switching”, shifting from one language to another in the same conversation with speech partners competent in the same languages. Another is "borrowing", the use of words from another language in order to better express the meaning sought. This is no sign of confusion, but the mobilization of a broader repertoire in a multilingual environment. In order to minimize involuntary mixing and confusion, it is advisable that each of the parents should always use the same language to their young children in order to give them a clear model.
Not necessarily. When comparing with monolingual children, some delays in activating one or more of the child’s languages are often reported. These delays are difficult to estimate precisely since even monolingual children do not all take the same time in activating their language.  Children need to "decipher" different codes at the same time and it is therefore normal that it may take them longer: they need to learn different words for each thing, different ways to combine them and when, with whom and how to use each of their language systems. This delay is generally not long but it may create some frustration for the children and also for their parents, eager to be reassured about the linguistic competence of their multilingual child. The different languages will generally blossom when the child is ready without creating any significant problem. On the contrary, changing the family languages after some months or years is generally not recommended. It is likely to confuse the child and may produce further delays on top of compromising the child’s bilingualism perhaps irreversibly.
Television and other media in languages other than the mother tongue are also an excellent support for bringing up multilingual children. The fact that the choices available on cable or satellite TV or on the internet are increasingly diverse is, in this respect, both a blessing and a curse for language learning. A blessing because of the variety of languages now accessible in this way. But also a curse because if the choice is left to the children the law of least effort will often lead them to select audiovisual consumption in the language in which they already feel most comfortable. Moreover, excessive TV watching with frequent zapping or highly fragmented children programmes can affect negatively children’s capacity for sustained attention. Hence, TV and the internet can be wonderful instruments in the service of multilingualism, but increasingly only if their use is sufficiently monitored by the parents. This is not easy in many family contexts. But it does not follow that there is no room for improvement. In particular, parents should encourage their children to watch broadcasts in the languages they need to learn with subtitles either in those languages or in their mother tongue. A significant positive impact can be expected both on their oral competence and on their ability to read.
The answers to the questions featuring under the five headings on our homepage express the convictions that underlie the Marnix Plan. Some of them are universally shared. Others are controversial. Well-documented objections are most welcome (at firstname.lastname@example.org) and could lead us to revise our views. The success of the Marnix Plan depends on its being firmly rooted in a lucid analysis of Brussels and the world as they are, of the opportunities they offer to our hopes but also of the obstacles they put in their way.
We are particularly grateful to Hugo Baetens Beardsmore, Aafke Buyl, Manon Buysse, Nicole Bya, Grégor Chapelle, Bastien De Clercq, Rudolf De Smet, Dany Etienne, Rudi Janssens, Kari Kivinen, Johan Leman, Silvia Lucchini, Jessica Mathy, Françoise Pissart, Hannelore Simoens, Marianne van de Graaff for extremely useful material and/or feedback, to Jolien De Paepe and Diederik Vandendriessche for the Dutch translation of the original English version, and to Sophie Dehareng and Amélie Lelangue for its translation into French.
The responsibility for the current formulation, however, is solely ours.
Alex Housen, Anna Sole-Mena, Philippe Van Parijs,
coordinators of the Marnix Plan.
 See the brochure published (in Dutch, French, English, Arabic, Turkish, Italian, Spanish and German) by the Onderwijscentrum Brussel for parents who want to send their children to a Dutch-language kindergarten or primary school.
 See, for example, for recent evidence involving French and Dutch, Do Children hearing two language acquire language at a slower rate?