Easily write in 9 Indian scripts with easy-to-use transliterate keyboard. Visitor/blog owner does not need to download any special software.
Make sure that in your theme in footer.php file wp_footer() function call exists.
This is not a plugin problem. Your database must have a Unicode(UTF-8) collation. If you have brand new blog, create a database with proper collation. If you have existing blog, try searching for a plugin which will convert your existing blog content into Unicode.