Julie Sedivy was borm in Czechoslovakia but emigrated to America as a small child. Writing at Language Log, she notes, "Like many immigrants, I’d learned my heritage language as a child within rather constrained domestic spheres and had never used it to negotiate cab fare or discuss existential concerns, let alone describe my professional activities." She goes on to assert:
As it turns out, a language is rarely truly forgotten, merely submerged.A number of intriguing studies reveal the cognitive remnants of previously-learned languages that have fallen into disuse. “Forgotten” languages appear to make their continued presence known primarily through re-learning; even when initial testing suggests that a language has been lost, those who have been exposed to it earlier in life often show dramatically accelerated re-learning. This has been observed in the domains of grammar, vocabulary, and particularly, phonology.
The rest of the post expands on that, mostly by talking about her own experience – she's currently in Czechoslovakia on a writing project. The comments (20 of them at the moment) are interesting as well.
This is interesting in itself, but also in the context of Dan Dennett's recent realization that, in fundamental ways, human brains are not like digital computers. The phenomenon Sedivy is describing, reactivating a forgotten/unused language, is not the sort to thing that happens with digital computers.