Take a boy who spent the first six years of his life in Japan, speaking Japanese. Then nine years in America, speaking English. Another year in America passes by, only filled with trauma that later caused hallucinations and variations on the 'mentally ill' theme. After that year passes, he's taken somewhere safe. As time goes by in that place, he often gets upset or confused and when this happens, he switches from English to Japanese at random, particularly during hallucinations, because of the way his mind works- it subconsciously goes from the language he hates (English) to the comforting language in which he spoke his first words.
Is there a term for this? If so- what in the name of Madonna is it?!
Edit: And the verdict is- *drumroll* language reversal and dissociation during regression! Seriously, much thanks to everyone who helped. I may actually be happier about figuring out this bit of my boywhore's mental state than I was when Atsushi covered I Like Chopin. ...No, really.