According to an article I found today, a study at the University of British Columbia found that babies as young 4 months old can tell when someone has switched from one language to another just from visual clues, such as the shapes and rhythm of the speaker’s mouth and face movements.
Babies aged 4, 6 and 8 months from monolingual and bilingual families were shown silent videos of people speaking sentences first in English, then in French. The 4 and 6 month old babies paid more attention to the videos and watched them longer when the languages switched, which indicates that they noticed differences. By age of 8 months, only babies raised in bilingual homes were able to tell the difference between the languages.
Perhaps it would be interesting to have a language quiz or two featuring silent videos of people speaking different languages and asking you to try to work out how many languages were being spoken and even which ones. What do you think?
If you have an suitable videos, or know where they are available, do let me know.