@Mayateacher - Okay, if i wanted you to repeat your above message in Hindi; should I say A) "हिंदी में बोलो", or B) "हिंदी बोलो"?
If you follow A) then you must convey your above message in Hindi. Whereas if you follow B) , you are free to say anything in Hindi... perhaps reading to us yesterday's Hindi newspaper :-)... you'd still be speaking Hindi.
So, hopefully you can appreciate A) and B) are not the same - contrary to what you said in your message.
I agree, the two are nearly synonymous, tho' the form "Speak in Hindi" weakly implies saying a specific thing in Hindi (as opposed to saying it in some other language), as opposed to speaking Hindi in general. Therefore both forms (with or without "in") should be accepted.