Multilingualism has likely been common throughout much of human history, and today most people in the world are multilingual. In tribal hunter-gatherer societies, multilingualism was common, as tribes must communicate with neighboring peoples and there is often intermarriage. In present-day areas such as Sub-Saharan Africa, where there is much variation in language over short distances, it is usual for anyone who has dealings outside their own town or village to know two or more languages.
When speakers of different languages interact closely, it is typical for their languages to influence each other. Languages normally develop by gradually accumulating dialectal differences until two dialects cease to be mutually intelligible, somewhat analogous to the species barrier in biology. Language contact can occur at language borders, between adstratum languages, or as the result of migration, with an intrusive language acting as either a superstratum or a substratum.
Language contact occurs in a variety of phenomena, including language convergence, borrowing, and relexification. The most common products are pidgins, creoles, code-switching, and mixed languages. Other hybrid languages, such as English, do not strictly fit into any of these categories.
Forms of influence of one language on another 
Borrowing of vocabulary 
The most common way that languages influence each other is the exchange of words. Much is made about the contemporary borrowing of English words into other languages, but this phenomenon is not new, nor is it very large by historical standards. The large-scale importation of words from Latin, French and other languages into English in the 16th and 17th centuries was more significant. Some languages have borrowed so much that they have become scarcely recognizable. Armenian borrowed so many words from Iranian languages, for example, that it was at first considered a branch of the Indo-Iranian languages. It was not recognized as an independent branch of the Indo-European languages for many decades.
Adoption of other language features 
The influence can go deeper, extending to the exchange of even basic characteristics of a language such as morphology and grammar. Nepal Bhasa, for example, spoken in Nepal, is a Sino-Tibetan language distantly related to Chinese, but has had so many centuries of contact with neighboring Indo-Iranian languages that it has even developed noun inflection, a trait typical of the Indo-European family but rare in Sino-Tibetan. It has absorbed features of grammar as well, such as verb tenses. Romanian was influenced by the Slavic languages spoken by neighboring tribes in the centuries after the fall of the Roman Empire, not only in vocabulary but also in phonology and morphology. English has a few phrases, adapted from French, in which the adjective follows the noun: court-martial, attorney-general, Lake Superior. It is easy to see how a word can diffuse from one language to another, but not as obvious how more basic features can do the same; nevertheless, this phenomenon is not rare.
Language shift 
The result of the contact of two languages can be the replacement of one by the other. This is most common when one language has a higher social position. This sometimes leads to language endangerment or extinction.
Stratal influence 
However, when language shift occurs, the language that is replaced (known as the substratum) can leave a profound impression on the replacing language (known as the superstratum), when people retain features of the substratum as they learn the new language and pass these features on to their children, leading to the development of a new variety. For example, the Latin that came to replace local languages in present-day France during Roman times was influenced by Gaulish and Germanic. The distinct pronunciation of the dialect of English spoken in Ireland comes partially from the influence of the substratum of Irish. Outside the Indo-European phylum, Coptic, the last stage of ancient Egyptian, is a substratum of Egyptian Arabic.
Creation of new languages: Creolization and mixed languages 
Language contact can also lead to the development of new languages when people without a common language interact closely, developing a pidgin, which may eventually become a full-fledged creole language through the process of creolization. A prime example of this is Saramaccan, spoken in Suriname, which has vocabulary mainly from Portuguese, English and Dutch, but phonology and even tones which are closer to African languages.
A much rarer but still observed process is the formation of mixed languages. Whereas creoles are formed by communities lacking a common language, mixed languages are formed by communities fluent in both languages. They tend to inherit much more of the complexity (grammatical, phonological, etc.) of their parent languages, whereas creoles begin as simple languages and then develop in complexity more independently. It is sometimes explained as bilingual communities that no longer identify with the cultures of either of the languages they speak, and seek to develop their own language as an expression of their own cultural uniqueness.
Mutual and non-mutual influence 
Change as a result of contact is often one-sided. Chinese, for instance, has had a profound effect on the development of Japanese, but the Chinese language remains relatively free of Japanese influence, other than some modern terms that were reborrowed after having been coined in Japan based on Chinese precepts and using Chinese characters. In India, Hindi and other native languages have been influenced by English up to the extent that loan words from English are part of day to day vocabulary. In some cases, language contact may lead to mutual exchange, although this exchange may be confined to a particular geographic region. For example, in Switzerland, the local French has been influenced by German, and vice-versa. In Scotland, the Scots language has been heavily influenced by English, and many Scots terms have been adopted into the regional English dialect.
Linguistic hegemony 
Obviously, a language's influence widens as its speakers grow in power. Chinese, Greek, Latin, Portuguese, French, Spanish, Arabic, Persian, Sanskrit, Russian, German and English have each seen periods of widespread importance, and have had varying degrees of influence on the native languages spoken in the areas over which they have held sway.
Dialectal and sub-cultural change 
Some forms of language contact affect only a particular segment of a speech community. Consequently, change may be manifested only in particular dialects, jargons, or registers. The South African dialect of English has been significantly affected by Afrikaans, in terms of lexis and pronunciation, but English as a whole has remained almost totally unaffected by Afrikaans. In some cases, a language develops an acrolect which contains elements of a more prestigious language. For example, in England during a large part of the Medieval period, upper-class speech was dramatically influenced by French, to the point that it often resembled a French dialect. A similar situation existed in Tsarist Russia, where the native Russian language was widely disparaged as barbaric and uncultured.
Sign languages 
Language contact is extremely common in most deaf communities, which are almost always located within a dominant oral language culture. It can also take place between two or more sign languages, in which case the expected contact phenomena occur — lexical borrowing, foreign "accent", interference, code switching, pidgins, creoles, and mixed systems. However, between a sign language and an oral language, while lexical borrowing and code switching also occur, the interface between the oral and signed modes produces unique phenomena: fingerspelling, fingerspelling/sign combination, initialisation, CODA talk, TDD conversation, mouthing and contact signing.
See also 
- Areal feature
- Language transfer
- Creole language
- Lingua franca
- Mixed language
- Phono-semantic matching
- Post-creole speech continuum
- Language island
- Lexical gap
- Linguistic anthropology
- http://www.cal.org/resources/Digest/digestglobal.html A Global Perspective on Bilingualism and Bilingual Education (1999), G. Richard Tucker, Carnegie Mellon University
- Hadzibeganovic, Tarik, Stauffer, Dietrich & Schulze, Christian (2008). Boundary effects in a three-state modified voter model for languages. Physica A: Statistical Mechanics and its Applications, 387(13), 3242–3252.
- Waterman, John (1976). A History of the German Language. University of Washington Press, p. 4
General references 
- Sarah Thomason and Terrence Kaufman, Language Contact, Creolization and Genetic Linguistics (University of California Press 1988).
- Sarah Thomason, Language Contact - An Introduction (Edinburgh University Press 2001).
- Uriel Weinreich, Languages in Contact (Mouton 1963).
- Donald Winford, An Introduction to Contact Linguistics (Blackwell 2002) ISBN 0-631-21251-5.
- Ghil'ad Zuckermann, Language Contact and Lexical Enrichment in Israeli Hebrew (Palgrave Macmillan 2003) ISBN 1-4039-1723-X.