Well, some of the characters in the document in the OP are directly cribbed from Japanese characters (there are recognisable, if slightly changed, te, ro and to characters from hiragana among others), and the more complex ones resemble kanji; in that way, the structure of the language seems to be basically Japanese. But I would agree that in lore, their language is probably not meant to resemble Japanese to that extent (I'm liking the idea about the snake-letters). I did pin more of a Chinese culture onto the Akaviri, because of the emphasis on tigers and dragons (Chinese dragons are also more snake-like than Western dragons). Not to mention the Tang Mo, which to me seem directly related to the Chinese Monkey legend, which is a brilliant story if you've never read it. The Tsaesci are described as naga, which come from Hindu/Buddhist mythology, and again Buddhism is the dominant religion of China.
I agree there are more influences than purely China to the Tsaesci but I would argue that as a whole land, China is the strongest influence for Akavir. I will admit though, I know little about either Akavir or China, so I could be totally wrong!
I've gone over that document a couple of times now, and I can't for the life of me recognize any Japanese characters. It all looks like snakes and dragons (check out that third-last letter on the second page- reminds me of the Empire's http://www.uesp.net/wiki/File:Akaviri_banner.jpg) to me.
Aztecs were preoccupied by alot of creatures, not just snakes... actually i think the jaguars and eagles were more important to them....
And besides that snakes are worshipped in many other cultures...
Akavir are heavely based on the Pacific Asians... just as much the Nords were based on... the Nords....
I'll agree with you about the Aztecs, that was mostly speculation on my part (hence the IMO), but I think it's foolish to say that Akavir is entirely based on Asia.
The Tsaesci, sure. To an extent, anyway. The others? Not so much. What do we know about the culture of Tang Mo, besides it being the "Thousand Monkey Isles"?
Like I said earlier, basically the only similarities with the Tsaesci and real-life Asian cultures are the vertical writing and the katanas. They don't wear armor, they drink Goblin blood, they look like golden snakes (as do their letters... there seems to be a thing for numbers here too, see the below creation myth as there's some mathematical things in that too) and they "http://www.imperial-library.info/obscure_text/tsaesci.shtml" which is probably a metaphor, but either way implies that they have a completely alien mindset that can't really be compared to any real-life human culture.
Oh, they're also immortal. Unless someone kills them.