The captcha image is created by the WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php file. line 2 of that file sets the characters that are used:
$useChars = '2345679bcdhkmnrstwxzADEFHJKLMNPRSTWXYZ';
you can change this to use the characters you wish. I'm not 100% sure you can use Chinese characters however, the would have to be supported by the font you choose.