If it's not easy to write the plug in. I just need a guide line. But can you please quickly tell me how to turn on the Chinese translation.

Actually I stand corrected.
This forum has several topics on posting modules outside index.php and inside parts of your template.

You can add a K2 User module on top of the comments form which will handle the login and the redirection of the user.

I will try what you said. Thank you.

Can you please also help me on how to turn on the Chinese language on K2.

Just install the Chinese language pack from getk2.org/extend/translations