As we know, we could add our own words in Hinglish and Hindi(Devanagari) keyboards in WP 8. But that feature was removed in WP 8.1. If you want to see that feature again in Windows Phone 8.1 or Windows 10( mobile) , please vote here :
https://windowsphone.uservoice.com/...-back-user-dictionary-in-hindi-and-hinglish-k
https://windowsphone.uservoice.com/...-back-user-dictionary-in-hindi-and-hinglish-k