when user log in opting hindi then he can type in hindi ,
For user to type in Hindi, his/her regional language settings needs to be set accordingly. For a web application, there is no control on the same.
You can design your web application to handle multiple languages but you cannot force someone to type into a specified language.
For multilingual website, have a look at these:
.NET - Localization using Resource file
Globalization and localization demystified in ASP.NET 2.0