Awesome write up Florae. It is really enlightening and educating.
As a matter of fact, FAS is one of the problems associated with damage to one of the areas involved with language; broca's area. This condition could be so annoying and devastating.
There's this other area involved with language too. It is called Wernicke's area. Damage to this area has a somewhat but not precisely opposite effect on the person as compared to the lesion to the Broca's area.
In this other case, the person can speak but not be able to understand what is spoken or written.

I am a bit curious here on how someone could just wake up and start speaking an entirely different language fluently. Is it that the new language is spoken well that a native of the language could understand it or is it just sounding like another perfect language to a native of the previous language?