bilingual kids can identify 3rd language is in use

http://www.sciencedaily.com/releases/2011/02/110218092539.htm

Infants raised in households where Spanish and Catalan are spoken can discriminate between English and French just by watching people speak, even though they have never been exposed to these new languages before, according to University of British Columbia psychologist Janet Werker.

http://www.sciencedaily.com/releases/2007/05/070524145058.htm

Researcher Whitney Weikum found that infants are able to discern when a different language is spoken by watching the shapes and rhythm of the speaker's mouth and face movements.