I am typing into my terminal and the stdin is changing and adding characters on its own
I don't have any problem with trying to paste that line into zsh running in Terminal.app.  Some testing...
zsh: processed normally
dash: processed normally
oksh: processed normally
pdksh: processed normally
tcsh: processed normally
bash: processed normally
fish: processed normally
ksh: takes the Chinese as editor commands and weirds out

I'll report that as a ksh bug, I suppose.
But your problem would seem to be OhMyZsh, not Terminal.app.  I can replicate the ksh behavior in iTerm as well.
Edit: ksh has a bug with multibyte characters and the KEYBD trap.  See https://github.com/ksh93/ksh/issues/307 and https://github.com/att/ast/issues/197