"Speak Hindi" is the same as "Speak in Hindi." In North America it is more common to say it the first way.
I had a similar thing on another question and suggested the no-"in" version as an alternative to, so hopefully they'll get around to accepting it.
Both phrases make sense in English (and I assume the above without में makes sense in Hindi too) and I've found that in Duolingo, if the direct translation makes sense it's the best option to go for. This sentence teaches a post-position as well.