In 3D WebView, all keyboard input is sent to a webview through the IWebView.HandleKeyboardInput() method like described in this article. You can pass Chinese, Japanese, and Korean characters to HandleKeyboardInput(), like this:
webViewPrefab.WebView.HandleKeyboardInput("猫");3D WebView's on-screen Keyboard component doesn't support Chinese, Japanese, or Korean, but you can use a 3rd party keyboard asset that supports those languages and then pass the keyboard's output to HandleKeyboardInput() as described in this article. 3D WebView doesn't have built-in support for IME (input method editor), which is commonly used to compose Chinese, Japanese, and Korean text. However, you can use a 3rd party keyboard or input field that supports IME and then pass the characters output by the IME-enabled keyboard or input field to HandleKeyboardInput(). I have a feature request to add support for IME on platforms that can support it (likely just Windows and macOS), and I will update this article in the future if I add support for that.