I figured hey.... lots of players are unclear on this, why not ask?

Can a wizard who uses the polymorph spell, talk normally, or with the limitations of their race?

So spiders not at all... zombies with great difficulty. Umber hulks, buzzing clicking noises. Trolls.... is their speech slured due to their mouths, or stupidity-which a polymorphed mage does not suffer from.

You have it, pretty much. If the animal is capable of speech, they can talk in the manner of the creature (eg, maybe a deep voice for trolls). If the creature is incapable of speech, they can't talk (eg, a rat would still squeak).

Their intelligence remains the same, I believe. If your stat doesn't decrease, neither does the ability.

So can you talk normally in troll form?

It would be low and gutteral, but yes.