First Language

A first language (also native language, mother tongue, arterial language, or L1) is the language(s) a person has learned from birth or within the critical period, or that a person speaks the best and so is often the basis for sociolinguistic identity. In some countries, the terms native language or mother tongue refer to the language of one's ethnic group rather than one's first language.
By contrast, a second language is any language that one speaks other than one's first language.
Terminology
Sometimes the term native language is used to indicate a language that a person is as proficient in as a native individual of that language's "base country", or as proficient as the average person who speaks no other language but that language.
Sometimes the term mother tongue or mother language is used for the language that a person learnt as a child at home (usually from their parents). Children growing up in bilingual homes can, according to this definition, have more than one mother tongue or native language.
In the context of population censuses conducted on the Canadian population, Statistics Canada defines mother tongue as "the first language learned at home in childhood and still understood by the individual at the time of the census." It is quite possible that the first language learned is no longer a speaker's dominant language. This includes young immigrant children, whose families have moved to a new linguistic environment, as well as people who learned their mother tongue as a young child at home (rather than the language of the majority of the community), who may have lost, in part or in totality, the language they first acquired (see language attrition).
Mother Language
The term mother language should not be interpreted to mean that it is the language of one's mother. In some paternal societies, the wife moves in with the husband and thus may have a different first language than the husband. Mother in this context originated from the use of "mother" to mean "origin" as in motherland.
In some countries such as Kenya, India, and various East Asian countries, "mother language" or "native language" is used to indicate the language of one's ethnic group, in both common and journalistic parlance (e.g. 'I have no apologies for not learning my mother tongue'), rather than one's first language. Also in Singapore, "mother tongue" refers to the language of one'sethnic group regardless of actual proficiency, while the "first language" refers to the English language that was established on the island through British colonisation, which is the lingua franca for most post-independence Singaporeans due to its use as the language of instruction in government schools and as a working language.
J. R. R. Tolkien in his 1955 lecture "English and Welsh" distinguishes the "native tongue" from the "cradle tongue," the latter being the language one happens to learn during early childhood, while one's true "native tongue" may be different, possibly determined by an inherited linguistic taste, and may later in life be discovered by a strong emotional affinity to a specific dialect (Tolkien personally confessed to such an affinity to the Middle English of the West Midlands in particular).
21 February has been proclaimed the International Mother Language Day by UNESCO on 17 November 1999.
Significance
The first language of a child is part of their personal, social and cultural identity.[4] Another impact of the first language is that it brings about the reflection and learning of successful social patterns of acting and speaking. It is basically responsible for differentiating the linguistic competence of acting.
On multilinguality
One can have two or more native languages, thus being a native bilingual or indeed multilingual. The order in which these languages are learned is not necessarily the order of proficiency. For instance, a French-speaking couple might have a daughter who learned French first, then English; but if she were to grow up in an English-speaking country, she would likely be proficient in English. Other examples are India and South Africa, where most people speak more than one language.
The Brazilian linguist Cleo Altenhofen considers the denomination "mother tongue" in its general usage to be imprecise and subject to various interpretations that are biased linguistically, especially with respect to bilingual children from ethnic minority groups. He cites his own experience as a bilingual speaker of Portuguese and Riograndenser Hunsrückisch, a German-rooted language brought to southern Brazil by the first German immigrants. In his case, like that of many children whose home language differs from the language of the environment (the 'official' language), it is debatable which language is one's 'mother tongue'. Many scholars[citation needed] have given definitions of 'mother tongue' through the years based on common usage, the emotional relation of the speaker towards the language, and even its dominance in relation to the environment. However, all of these criteria lack precision.
Defining mother tongue
 Based on origin: the language(s) one learned first (the language(s) in which one has established the first long-lasting verbal contacts).
 Based on internal identification: the language(s) one identifies with/as a speaker of;
 Based on external identification: the language(s) one is identified with/as a speaker of, by others.
 Based on competence: the language(s) one knows best.
 Based on function: the language(s) one uses most.


Sources: encyclopedia2.thefreedictionary.com/First+language+acquisition-01.04.2011