Early bilinguism favors adaption skills and cognitive aptitudes. The psycholinguist Ranka Bijeljac-Babic, member of the Laboratoire de psychologie de la perception de Paris-Descartes, and author of L’enfant Bilingue, she studies early bilinguism.
Many recent studies state the advantages of bilinguism on the cognitive development of the children. Ranka Bijeljac-Babic highlights how enriching is a double culture, as early as possible. She insists that this bilinguism is not only considering English or an international language. A five-month-old baby is able to discriminate languages.
Both languages are never developed at the same path. A language is always dominant, even though the dominant language can change along their childhood. Thus, there is no perfect bilinguism. Ms. Bijeljac-Babic says that a double culture does not slow down the acquisition of language. She also mentions that believing the traditional one locator/one language is an incorrect stereotype, and does not create any linguistic chaos.
Speaking two languages as soon as possible is also positive as the more languages you speak, the easiest it is to learn new languages. The only real worry she could mention is the risk of “semi-linguism”, when no language is really mastered.
Last but not least, the constant change of language favors attention to environment changes. Bilinguism is never easy, but certainly is a positive personal and social advantage. Listening to the podcast (in French): https://www.franceinter.fr/emissions/la-tete-au-carre/la-tete-au-carre-13-fevrier-2017