Vygotskian idea of externalization/internalization should not be taken as input from the outside world simply being put into one's mind.
[I]t [ =externalization/internalization] is the appropriation of ideas that assimilate with pre-existing knowledge to reconstruct one’s understanding. (p. 2)
Perhaps more important is the issue of "human agency" (Lantolf & Thorne, 2006).
Human agency entails voluntary control over one’s actions as well as the ability to assign relevance to events, and is culturally, socially, and historically mediated. The significance of human agency is twofold: first, humans can decide what they want to appropriate and what they want to ignore; second, what they decide is a product of their history. In other words, internalization and transformation is individual, based on participation in social activities, and gauged by how these social activities are manifest in thought and in activities. (p. 2)
Lantolf, J.P. and Thorne, S.L. (2006). Sociocultural theory and the genesis of second language development. New York: Oxford University Press.