The proper model for foreign-language teaching should really have been not first-language acquisition but rather the natural acquisition of a second language. Numerous studies of children growing up with two languages in the family have shown that they employ both languages in such a way that the one is used as a help for the other. If, for instance, the child wants to phone its grandparents in France to tell them about something which it has not yet processed in French, it will first get help by asking “Comment dit-on, I cut my finger?” The lack of vocabulary is solved in the most easily conceivable way (Kielhöfer & Jonekeit, 1983). Requests for linguistic assistance take different forms, they are the rule, not the exception. Also, bilingual speakers often feel the need to reassure themselves in their stronger language. I find those examples most convincing where the children provide for themselves translations which have been deliberately withheld from them.
Source: We only learn language once. The role of the mother tongue in FL classrooms: death of a dogma.