I'm a bit confused now. Didn't you say in
this post that the Kindle supports Devanagari???
When creating your own Kindle books please remember that only the new KF8 format (AZW3 files) support some Indic languages.
I.e., if you use Calibre make sure to select AZW3 as the output format or use Kindle Previewer, which will automatically generate both formats. Also transfer files via USB/Calibre do not use Kindle e-mail.