More than half the world is bilingual or multilingual. So when growing up exposed to two (or more) languages at once, children have two systems to learn, and they must also learn when to speak each language. The choices here depend on who the addressee is, and on the setting. Exposure to the two languages may be uneven, and also vary over time, depending on who the child spends time with. Choice of language depends on common ground, on the topic, and on the language common to the child’s conversational partners. The early stages of acquisition are very similar, from perception of sounds and sound sequences to early babbling; from comprehension of words to attempts to produce them. Early vocabularies contain many doublets, freely accumulated as children learn more of each language. (This is consistent with contrast, but not with mutual exclusivity.) Language mixing tends to mirror adult usage and so varies across languages. Children attend not only to differences in the sound systems but also to structural differences of all kinds. Conversational skills develop in similar ways across languages, depending on exposure and practice, with language dominance fluctuating over one’s lifetime. Acquiring two dialects involves similar skills.

Bilinguals change their way of communicating when they are with monolinguals and when they are with bilinguals who share their languages. Whereas they avoid using their other language(s) with monolinguals (they are in a monolingual mode), they may call upon it (or them) when interacting with bilinguals, either by changing over completely to the other language(s) or by bringing elements of the other language(s) into the language they are speaking (they are then in a bilingual mode). The author reviews how he developed the notion of language mode, which, at the cognitive level, implies a change of activation of the languages and processing mechanisms. He summarizes some of the basic elements of language mode as described in a 2001 seminal chapter, discusses how language mode has fared since, and presents additional evidence for it. He then proposes some follow-up comments that deal with the level of activation of the deactivated language(s) in a monolingual mode, the complex nature of the variable that is language mode, and how it compares to the adaptive control hypothesis. He ends with reactions to language mode – many positive and some critical.

This chapter discusses the possible cognitive and socio-cognitive benefits children may enjoy due to multilingualism. We argue that children acquire advanced communication skills through everyday engagement in multilingual interaction with their family members and surroundings from the earliest stages of development. We outline the major findings on the nature of multilingual language development, parental input, and parent–child interactions in multilingual families to understand the context in which multilingual children may acquire their assumed advanced abilities in communication. We provide an extensive overview of current research scrutinizing the abilities multilingual children develop in the earliest years of life. This review covers cases of multilingual first language acquisition, which refers to the acquisition of two or more languages from before the age of 3, a cut-off age used to distinguish between simultaneous and successive development of multiple languages. The review draws on studies from both bilingual and multilingual (trilingual) first language acquisition, as many of the studies of bilingual children preceded those of multilingual children and can give meaningful insight into multilingual development.

Language contact is at its most intense within one and the same individual. This chapter discusses the dynamic and multifaceted phenomenon of individual bilingualism, which emerges when individuals learn to understand multiple language varieties. Individual bilingual language use may contribute to societal processes of language change, language maintenance, and language loss. It is as yet not fully clear how bilingual individuals affect such larger processes, but cross-linguistic influence in comprehension and production, patterns of language choice, and variable levels of proficiency across the lifespan all play a role. These in turn depend on individuals' contexts for learning and using languages throughout the lifespan. The chapter exemplifies some of these on the basis of bilinguals' language biographies (including Frederick the Great's). Language learning histories and opportunities for using each language help explain the large variability in language skills and use among bilinguals. New languages can be learned until well into adulthood. Their number is constrained only by learning opportunities and motivation. Previously learned languages, including languages learned very early in life, can be lost through lack of use and practice. Language attitudes play a large role in all of this.

The purpose of the chapter is: first, to provide an overview of the socio-linguistic landscape of the late antique Mediterranean as a historical setting in which early Christian writers lived, worked, and developed their ideas on languages and religious identities; the linguistic and social practices they observed and experienced everyday set up an important context, even though their theoretical speculations may not directly reflect realities on the ground. Second, to briefly account for Christian encounters with foreign cultures and languages as the religion took off and to inquire into how the socio-linguistic situation in the ancient Mediterranean influenced the development of Christianity in its nascent stage and how Christianity in turn affected linguistic processes in the region. The spread of Christianity among various peoples and ethnic groups within and without the gradually disintegrating Roman empire was concurrent with complex formative processes within Christianity itself and took place amidst Trinitarian and Christological debates. The survey problematizes the role languages and linguistic distinctions played in these controversies and indicates several loci of potential tensions to be discussed further in the book (the alleged links between Punic speakers and Donatists, Germanic speakers and Arians, Syriac speakers and followers of non-Chalcedonian doctrines).

The most frequently expressed concern is whether early bilinguals will be able to differentiate their languages. Research on child bilingualism has demonstrated that this is indeed the case. Children acquiring two languages simultaneously are able to differentiate their lexical and grammatical systems from very early on. Ocassional language mixing is not an indication of an underlying unitary system, fusing two or more linguistic competences. Rather, mixing is a performance phenomenon. Most mixed utterances are instances of code-switching, i.e. linguistic behaviour constrained by grammatical and sociolinguistic principles. Early mixes may also result from a choice of language that is unexpected from an adult perspective. Choosing the adequate language in bilingual settings requires sociolinguistic knowledge that is acquired in the course of children’s socialization. Parents can support bilingual L1 acquisition by their own linguistic behaviour. Following the ‘one person, one language’ (OPOL) principle is a method that has been applied successfully for over 100 years.