Q: When new pandaren are created they are all neutral. Will they lose their ability to communicate with other Pandaren of the other faction once they have chosen a side?
A: Pandaren will not be able to speak to pandaren from other factions.
My question is, why? From a lore standpoint this makes absolutely no sense. Pandaren are there to be the peacemongers and I am quite sure that once you choose a faction there's not some magical language barrier that gets cast on the members of said faction that would prevent you from speaking your native language. From a purely technical standpoint, and maybe from a PvP battleground standpoint, I might understand. Maybe.
If I click the chat bubble and find "Pandanese" or whatever their language is supposed to be, then we should be able to /say or /yell hi to each other. Given the nature of the Pandaren and what was said about them in the Q&A sessions there's just no real reason for it except for someone's asinine desire to absolutely prevent cross-faction talking. To restrict even members of the exact same race who set off from their home land on separate paths at the same time from being able to talk to each other is far more of a nuisance than anything else.
TL;DR: Pandarens not being able to talk to each other cross-faction makes no sense. While a small detail, is quite story breaking especially because of the Pandaren's main purpose for being there in the first place.