The Hunspell spell-checker used by Firefox will be introduced to Pegasus Mail in its new version V5 later this year. Dictionarys including Hindi are available for it.
I am not sure about word direction detection, ie right-to-left rather than default left-to-right word sampling. Information on this will bcome available closer to introduction. See http://wiki.services.openoffice.org/wiki/Dictionaries for dictionaries available.