I spoke not of the ability to differentiate between languages, but about the ability to acquire language(s). If a child of a sensitive age acquires more than one language, they "are located" in the same brain area. A person past the sensitive age has "to create" a different locus for every new language.
As for an infant's ability to differentiate between languages, I wouldn't make such a bold statement. I would rather say that a child switches from one language to another depending on which of the parents he is talking to. I myself talk to my mother mostly in Ukrainian and to my father mostly in Russian.
If a child is addressed to by one parent, but in different languages he is likely to lump the languages together. I witnessed it when I saw one of my colleague's daughter whom she was trying to teach English from the earliest age possible (that is besides her mother tongue). As a result the child used English word roots in combination with Russian suffixes.
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