In my Unity Game, There is an input field for entering Name of the user. In that input field, I need to restrict the user from typing name in other languages. Only English should be allowed to type into the Input Field. Is there any way to do this? For example, if a player selects Malayalam or any other language and types into the input field using the keyboard, I need to show a message to the user to use only names in English. Can this be achieved in Unity?
You can only restrict accept characters. So, you could disallow non-English characters from the input. However, if you could spell a name in another language, in English, that’s impossible to prevent, right? By what measure could you know? 
There is a UnityEvent/callback for validating characters, for inputfield, I believe.
The problem is my native language Malayalam is not rendering properly in Unity Android, and in Unity iOS its showing as square boxes. So what should be done to solve this?
Oh, I see. That I’m unsure of. You may need to get a font that supports your language, if the one in Unity by default does not. Sorry, I didn’t realize that that’s what you had meant. I do know you can add fonts, but I wouldn’t be very good at giving advice to doing so. Add a small label above the textbox/inputfield that says (Please use English) ?
(you could keep it showing always or only when they’re entering their name, depending on your setup)
You’ll have to download a font that has the Malayalam characters. I’ve found them here: Noto Home - Google Fonts