Reminds me of something I read is a psych class once, about this woman who had an accident which and woke up speaking German. Apparently, L1(language(s) learned as young children) and L2 are stored in different places in the brain. When the place where L1 was stored was damaged beyond repair, she only had L2. She had taken a German class in high school, and even though she had forgotten most of it, part of her brain remembered it, and so she could only speak in (high school-level, I assume) German.
I remember thinking about how I'd taken French (mostly forgotten now) and how creepy it would be if I could only speak in basic French, Japanese would be ok I guess, but basic French...