By just four months of age, infants can tell if a speaker is using a different languages, purely by the movement of their lips (i.e. no sound).
This study shows that 4- and 6-month-old infants can discriminate languages (English from French) just from viewing silently presented articulations. By the age of 8 months, only bilingual (French-English) infants succeed at this task. These findings reveal a surprisingly early preparedness for visual language discrimination and highlight infants’ selectivity for retaining only necessary perceptual sensitivities
ScienceDaily has more info here.