I am typing into my terminal and the stdin is changing and adding characters on its own I am using curl on my Macos Catalina terminal app, and the text I enter in stdin gets altered on me pressing enter Enter. For a simple example:
curl -d &quot; 印度是最好的 The weather is good&quot; -X POST http://localhost:8000/questions/21/
curl: (1) Protocol &quot;hhttp&quot; not supported or disabled in libcurl

is actually entered as :
 curl -d &quot; 印度是最好的 The weather is good&quot; -X POST hhttp://localhost:8000/questions/21/

, which I can see from previous command on terminal Control + P.
P.S - I've entered the command and thus, it shows libcurl error.
For an english sentence - you can see the error below:
terminal error
I have no idea why this is happening, I'm using zsh shell and ohmyzsh along with it - and this is happening the first time to me.
Adding an image here too, for further investigation.
terminal typing problems
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