In recent years, researchers have learned a lot about how the brain functions in the context of multilingualism. For example, the brain immediately distinguishes whether a language that is heard is to be categorised as the foreign language or the mother tongue. It has also been found, for example, that in most right-handed people, the left brain is responsible for language learning.
Nevertheless, research in the field of neurolinguistics is still in its infancy. It is therefore not yet possible to answer whether the organisation of language and linguistic processes work differently in bilingual or multilingual people than in monolingual people. However, based on what we know from findings to date, there are no indications that bilingualism or multilingualism would have a negative effect on the brain's functioning.
What has already been researched, however, is that children have demonstrably different biological requirements for language acquisition. They differ primarily in the speed at which they learn a second language. "The speed of language acquisition is only partly related to a child's mental faculties. There is only a small correlation between the pace of learning one or two languages and success in school. Thus, fast "learners" of languages in childhood are not necessarily more successful in adult life (Cathomas and Carigiet, p. 18)." The successful development of two or more languages is fundamentally dependent on the talent, ability and personality of the child, but also on the environment providing the stimulation (parents, relatives, friends, school, etc.).
It is also known that new languages are not learned at the cost of, but, rather, with the help of the mother tongue. After all, you don't acquire the rule of three at the expense of addition. First of all, the brain is not a speech bucket, but rather actively chooses at what point in time which knowledge is to be retained and called up. Secondly, the human brain has virtually indefinite capacity. The different languages therefore do not stand in each other's way, but rather complement each other. Today, it is assumed that the mother tongue even profits greatly from learning a new language, and that the individual languages have a positive influence on each other.
We can use the "iceberg image" of the Irish–Canadian linguist Jim Cummins to explain this. "As with an iceberg with two visible tips, bilingualism also has a visible linguistic surface that corresponds to the linguistic proficiency that is displayed in the language in question. However, beneath this visible part – beneath the surface of the water, as it were – there is a significantly larger part of "invisible" aspects of linguistic proficiency. The first visible or audible aspect of linguistic proficiency that we usually encounter in a second language is pronunciation. This is what influences our impression of the proficiency of our counterpart the most, even though it represents only a very small part of one's overall linguistic proficiency. When learning a second or third language in this "invisible area", the brain now draws on the knowledge that has been built up in the first language, on that "shared brain capacity" (Cathomas and Carigiet, p. 22)." So, for example, when a German-speaking child learns the word "train", only the form of the English word "train" has to be learned, but not the semantics (the meaning). This way, the image of "train" that has been stored in the first language can be easily used for the acquisition.
Source: Cathomas, Rico und Carigiet, Werner. Top-Chance Mehrsprachigkeit. Zwei- und mehrsprachige Erziehung in Familie und Schule (2008, 1. Aufl.), p. 12-27.