I was about to tell you this joke about carrots... but I know it's not grate...
Location isn't the only function of Locative, and not all prepositions denoting location take Locative.
I know this could not be logical speaking real life but could this be translated as "you are speaking with vegetables?"
No, wrong preposition (and therefore wrong case as well). That would be "z warzywami".
Oh wait, and that would be "Rozmawiacie" as well. After all "speaking with" is a conversation ;)
I read in another book that mówic requires dative. Is this true? Obviously using o changes the case.
why is this phrasing (to speak about) everywhere on duolingo? "to talk about" is much more natural. it makes me grunt every time
Because the most direct translation of "to talk about" is "rozmawiać", which is basically "to have a conversation". "speak" is at least less ambiguous, even if - I agree - less natural.