People can be called multilingual (or plurilingual) if they know two or more languages, whatever the level of competence. Some may know two or more languages so well that they may pass as monolingual native speakers of each of their respective languages in all situations and under all circumstances. Such cases are extremely rare. Most multilingual people master their various languages to different degrees. Moreover, what can be considered their stronger or weaker language may change from one context to another and in the course of their lives. Some multilingual people may be better in one language (for example, the French or Dutch they learned at school) for writing formal texts but better in another, even one they cannot write (for example, their Flemish or Moroccan dialect), for talking casually to relatives, expressing emotions or telling jokes. Thus, multilingualism comes in many shades. Ultimately, whether you are multilingual and how multilingual you are depends on how you perceive yourself and on whether you have the will to put into practice the competence you possess.
Some communities of people — countries, municipalities, schools, etc. — are officially monolingual, bilingual or multilingual, in the sense that formal communication is conducted in one, two or more languages. However, this official status need not reflect the number of native languages or of languages actually known or used by the members of the communities concerned. For example, the Region of Brussels Capital is officially bilingual French-Dutch, but based on the most recent data (see Multilingual Brussels), one can conjecture that Brussels residents have hundreds of distinct native languages and keep speaking them regularly. This multiplicity of native languages or of languages known by the members of a community — whether a family, an organization, a firm, a school, a neighbourhood, a town, a region or a country — can be called linguistic diversity. Communities that are officially multilingual or in reality linguistically diverse may vary greatly in the extent to which their members are multilingual. At one extreme, the various language groups that make up the community are unable to communicate with one another. At the other extreme, they are all fluent in each other’s languages. A truly multilingual community is one whose members are themselves multilingual. Brussels will be truly multilingual when all its residents will be multilingual to a varying but significant degree.
The consensus today is that multilingualism is extremely common, both for individuals and for communities. Even seemingly monolingual individuals and communities often turn out to be multilingual on closer scrutiny. According to the most reliable guesses, over half of mankind uses at least one language other than their first language at some point in their lives and to varying degrees of proficiency. According to the best available estimates (see www.ethnologue.com), there are about 7000 languages currently spoken, or at least understood, in the world. With only about 200 sovereign countries, simple arithmetic tells us that even if each language was confined to a single country, many countries would need to host a high degree of linguistic diversity and hence also, if they are to function at all, to have many multilingual citizens.
Until not so long ago, many in Europe believed that trying to master more than one language, especially languages less “distinguished” than Latin, Greek, French or German, would confuse the mind, make people asocial and even lead to criminal behaviour and mental illness. As a result of considerable research in the last few decennia, this view is no longer held by anyone in the scientific community. Instead, multilingualism is now widely regarded as an asset, for two main reasons.
The most obvious advantage of multilingualism for individuals is that it amplifies their communication potential. This is relevant in many contexts. Professionally, competence in more than one language often increases the probability of finding a job or of accessing better jobs. Culturally, being multilingual facilitates the discovery of different cultures, their literatures and traditions. Socially, it opens up the possibility of richer contacts both when travelling abroad and with fellow members of one’s own multilingual community and thereby tends to foster mutual understanding, tolerance and trust. In the case of families with foreign or mixed origins, it has the further advantage of enabling children to maintain an intimate bond with their extended families and a strong connection with their cultural roots, without this preventing them from integrating into the local community.
In addition to this communicative advantage, a growing number of recent studies have documented a range of cognitive and even neurological advantages of early multilingualism if properly managed. According to these studies, multilingual children tend to score better in terms of mental flexibility, creativity and analysis, and they tend to keep these advantages later in life. Other recent studies have shown that, owing to their increased neuro-cognitive flexibility, multilinguals are significantly less prone to develop dementia related symptoms such as Alzheimer, or they develop them at a later age and suffer from them to a less severe extent than monolinguals do. 
A possible disadvantage of multilingualism is that multilinguals’ mastery of each of their respective languages, when measured separately, tends to be lower than that of monolingual speakers of these languages. For instance, multilinguals may have a smaller active vocabulary in each of their languages than monolinguals. Also the influence of their other languages may seep through when multilinguals speak or write in one of their languages, and cause them to have an accent or make occasional grammatical errors. (This phenomenon is known as interference). Obviously, when taken together, the total linguistic knowledge and communicative abilities of multilinguals typically far outweigh those of monolinguals.
The main disadvantage of multilingualism for individuals is the time and effort it takes to learn a new language, particularly when developed later in life. [See Is there a minimum and maximum age for learning a new language?]
In the 19th and much of the 20th century, many held the view that multilingualism was bad not only for individuals, but also for communities. In the ideal nation-state, so they believed, the citizens should share one language and eradicate all others, for linguistic diversity and the multilingualism it generates could corrupt the soul of the nation, undermine the dynamism of its cultural life, threaten civic peace and national unity, and hamper economic development.
Today, the conjunction of globalization and migration has forced us to accept linguistic diversity as a central and irreversible fact of 21st century communities. In the worst case, linguistic diversity can generate a fragmentation of the community into sub-communities that are unable to communicate with each other or to share a common project. More generally, one must acknowledge running the economic, social and political life of a linguistically diverse community is often more difficult, more laborious, more conflict-ridden than that of a monolingual one. Yet, some countries, such as Singapore, Luxembourg and Switzerland, are proving that linguistic diversity is consistent with featuring among the five most prosperous countries in the world.
Multilingualism — the widespread learning of several languages by the members of a community — is the glue, the network of bonds that can turn linguistic diversity from a handicap into an asset. In particular, providing there is sufficient competence in shared local languages, the lasting presence of people with a large number of different native tongues in a place like Brussels offers not only a wonderful opportunity for the local exploration and appreciation of a great variety of cultures, but also a wealth of economically valuable connections with countless places in the world. Hence the importance of promoting multilingualism through the learning of linking languages — in the Brussels case, French, Dutch and English — while at the same time cherishing and maintaining as many as possible of the other languages in the community.
In a globalized world, knowledge of other languages is essential for enterprises that want to operate internationally. As a lingua franca in a large part of the world, English is an obvious choice as a common language of communication. However, being able to communicate in more languages gives a firm a competitive advantage over its rivals. Not only do certain countries use a lingua franca other than English (e.g. French, Spanish, Russian and Chinese). An enterprise can also increase its success on foreign markets by using its customers’ native languages. Doing so expresses respect towards the customers’ culture and identity, fosters trust and goodwill, proves the enterprise’s long-term commitment, helps it gain a better insight into local legislation and customs, and enables it to make its market research and advertising campaigns more effective. Moreover, operating in several languages makes it possible to recruit from a larger pool of candidates and to make its workforce potentially more dynamic by making it more diverse (see http://ec.europa.eu/languages/langu...). These various advantages accrue to enterprises operating globally. In the linguistically diverse Brussels context, however, they also accrue to enterprises that operate locally: a multilingual staff enables them to create a stronger bond with their customers and other stakeholders.
Respect for Europe’s linguistic diversity has been a major concern throughout the process of European integration and now features explicitly in Article 22 of the Lisbon Treaty (2007). For a linguistically diverse Union to work, it is obviously indispensable that enough of its citizens should learn other languages. It is therefore not surprising that European institutions keep emphasizing the importance of multilingualism for taking full advantage of the opportunities created by European integration. 
More specifically, the European Commission’s Action Plan of 2004 formulated the objective in terms of “mother tongue plus two”. In a 2008 report endorsed by the Commission , the idea was refined: each European citizen should acquire, in addition to English — the unnamed but irresistible lingua franca —, a “personal adoptive language” chosen among European languages because of some special affinity.
Whereas bilingualism with English is spreading fast across the European continent , the more ambitious objective of trilingualism is far from reached and may be unreachable in many regions of the European Union. However, in Brussels, the EU’s unofficial capital, the proximity of Dutch-speaking Flanders and French-speaking Wallonia, combined with the presence of an international community that operates increasingly in English, should make the European ideal a more realistic goal than in most other places.
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.
Why Bilinguals Are Smarter (2012) by Yudhihit Bhattacharjee, in New York Times, 18 March 2012
Bilingualism: consequences for mind and brain (2012) by E. Bialystok, FI Craik, G. Luk, in Trends in Cognitive Science, 16(4), p. 240-50.
Bilingualism: the good, the bad and the indifferent (2009) by E. Bialystok, in Bilingualism: Language and Cognition, 12, p. 3-11
Study on the Contribution of Multilingualism to Creativity (2009) by
 See A Rewarding Challenge. How the Multiplicity of Languages Could Strengthen Europe, by Amin Maalouf et al. (2008). Brussels: European Commission.