In short, a "superuser" is a user who can create other users.
But if the user is not super user, he is not allowed to install the language 'plpgsql' and 'pltcl' for database.
But my problem was the language is already installed but getting error on creation of the function.
And if any one is creating this function who is not owner of database, this problem occurs.
Thanks
Dinesh Pandey