Because they aren't adding any new content to existing civs, they probably won't feature them in the demo. Instead, two or even all three Asian civs will appear on a new Asian map (1 v 1) or campaign. For the campaign in particular, you will play one civ and make allegiances with the two others (e.g. China forging alliances with India or sending Buddhist scholars to Japan) or ally with one and battle the other, or battle each other in turn. It's also possible that the campaign deals with relations between an Asian civ and a Euro one (Dutch East India Company, Portuguese vs. Japan). My guess is they want to cram as much new material in the demo as possible, so the campaigns will be mostly Asian content.
As far as the "star" civ, the one whose cause you champion in the campaign, it will probably be China, because of its geographical middle ground between the other civs, its active diplomatic stance during this era, and general public familiarity with Ming-Qing dynasty history. I suppose the other two civs have star potential too, due to popular history and an equal billing as novel, unique civs. I suspect Japan's military will be novel and unconventional to use at first, this is both a plus and a minus in a demo civ; and India's villager economy is unusual (although I suspect China's will be even more so). If I had to choose, being a new player, I'd rather start out with a civ whose economy is relatively straightforward, so I could build quickly and focus my attention on all the new and unique units, techs, and strategies.
Crunkatog on ESO
Bart331 balance suggestion: aztec: remove civ
Voltiguer: Ender, Sioux in 1.04 will be a top civ, no matter how many layers of Sioux goggles you put on
schildpad on Elephants: ...their mansabdar unit sucks so hard it looks like a black hole
Crunkatog on Steam.