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, Malaysia 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 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.
Read more about this topic: First Language