Forgetting and learning languages

Regarding your comment on confusing the "weak" languages, i.e. languages which are at an early stage of ones learning proces - I can totally relate to that. When I was learning spanish the first three months, I would sometimes unintentionally mix german and spanish when trying to speak german, becuase both of those languages at that time, especially germen, where my "weak" languages.

At first, I would say something like: "Ich habe ein coche" hehe. It sounds silly, but I guess it supports your theory on how the brain works.
Nowadays, I dont confuse spanish when I try to speak german, maybe because my spanish is so much stronger now. I would never, for instance, confuse danish, nor english, if attempting to speak german, because those two are by far my strongest languages.