Awesome to see the "birds and worms" script get a shout-out. I've always thought it's extremely pretty. Of course, with Chu being a dominant empire, at least for the time being, the script may well get a boost. I'm not sure if it can work out timeframe-wise, but how cool would it be for Japan - or Nihon, rather - to base its own script on the "birds and worms" script, borrowing from China as in OTL, but just in a different way?
Very cool indeed, and I can see Korean scholars like Ho Seung'eun to pass on a modified version of the "birds and worms", maybe even some sort of syllabic or phonemic derivation.
What was the language spoken by the people of Nihon (I mean south of the Emishi)? Was it related to the language of Mahan?
Those are simply Japanese and Korean, though probably derived from some slightly different dialects. I haven't worked out their differences, so if I need to translate something I'll just use OTL Japanese and Korean. As for being related, well, there are some linguists who speculate they're part of the Altaic macro-family along with Turkish and Mongolian, but that's controversial to say the least.