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There's a huge number of Pokemon who can communicate with humans, including but not limited to all legenaries/mythicals, and nearly every Pokemon can understand human speech, the only exceptions being extremely dumb ones like Rhydon.
So your common Pokemon wouldn't be able to say "yes" in human terms but can at least consent be it non verbally or via telepathy. Even then it seems like nearly any Pokemon can learn human speech if they give up their ability to battle like Meowth in the anime.
