Did anybody make the connection of Sun bonking her head on the tree with Desmond getting smacked by the cricket bat during "Flashes Before Your Eyes?" When Desmond got hit in the head, his consciousness fully shifted from the "alternate" 1996 back to 2004. So it would appear that blunt trauma to the head causes your brain to shift conciousness between time periods.
But is it not feasible to think that it could also cause a shift in consciousness between timelines? After Sun hits her head on the tree branch after running away from the Man in Black, she suddenly doesn't know English anymore (I believe this was around the same time we saw her looking at herself really confusedly in the mirror in the hotel...I don't need to go over the significance of mirrors in pretty much everybody's flash-sideways). Is it possible that when she knocked herself on the branch, some of her consciousness from the FST (i.e. the part where she doesn't know any English, because she had no reason to leave Jin and learn English) spilled over into the original timeline? The importance of Sun's loss of language has to be more important than what is on the surface here. Possibly because she's not special like our favorite Scotsman, getting hit in the head doesn't cause her to completely shift her consciousness, or you could chalk it up it's gonna take a bigger branch for someone's consciousness to shift between 2 different timelines completely.
I know other people have commented on the possibility of Sun's aphasia going a little deeper, but I don't know if people caught the connection with Desmond. Lest we forget that Locke also lost the ability to speak after the turning of the failsafe key, the thing that caused Desmond's consciousness in the first place back to '96. Juliet also seemed to be shifting consciousness after receiving a huge dose of electromagnetic energy at the bottom of the Swan shaft.
So whaddya think?