Check this really cool tinyMCE extension to make your Joomla post content to appear in Hindi. It supports as many as Eight Indian languages including Devanagari(Hindi), Gurumukhi(Punjabi) and Bengoli. It is really easy to install and get going.
You can find instructions on how to install and make it work here http://amiworks.co.in/talk/akindicplugin-for-tinymce-and-joomla. With the help of information found here you could even extend it to make your menus and site text appear in Hindi or in your chosen language (check conversation on the referred page in detail). This really is a cool tool!