Cheers for the info - but how is "you speak about vegetables" an answer to a question? Seems you have to make a pretty big assumption that the solution here is an answer to a question.
What 4d1n means is that in Polish, even the natives sometimes 'ask questions' to figure out which case is needed and what is the right form. And the basic questions that Locative answers to are "o kim? o czym?" (about whom? about what?) which don't really have much to do with the name "Locative".
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.
"You speak about vegetables" isn't correct, it should be 'You are speaking/talking about the vegetables', or 'Are you speaking/talking about the vegetables?'