Good evening! So, in the first sentence, "sweet" is an adverb and it would be more correct if you write "You smile sweetly", although we can't say "You smile sweet" is wrong. Actually, we also use adjectives as adverbs in the Portuguese language when we are being informal. Anyway, "You smile sweet" can be translated as "Você sorri doce" and in the standard form "You smile sweetly", "Você sorri docemente". As you can observe that "-mente" is a sufix like "-ly": both indicates an adverb, generally. And in "Your smile is sweet", "sweet" is an adjective. You can translate it as "Seu sorriso é doce". There is an interesting thing that makes me agree with Mario. "Sweet" in English is literally "candy" and it means something like "food made of sugar" just like in Portuguese ("doce"). Although, when you used it like in these sentences you showed us, you doesn't mean that the person's smile is a candy or that is made of sugar. You mean it FIGURATIVELY, just like we do in the Portuguese language. So, "Seu sorriso é doce" and "Você sorri doce(mente)" are correct (and it is actually much more poetic... and sweet), but if you want to mean this in a literally way you should say "Seu sorriso é meigo" just as Mario has proposed before.

"You smile sweet" can't be decently translated into portuguese. "Your smile is sweet" = "Seu sorriso é doce", but we rarely use sweet for that. If will say that for a woman, I suggest "Seu sorriso é meigo".

Both ways are feasible to express pleasant smile. This is a figurative way of praising the smile. Her smile is sweet = Seu sorriso é doce. Sweet smile= Sorriso doce.

YOU SMILE SWEET = Voce sorri docemente. YOUR SMILE IS SWEET = Voce tem um doce sorriso. OBS: Lembre-se que nao podemos traduzir estas expressoes ao pe da letra. Elas tem que fazer sentido na nossa lingua, na nossa realidade. Abraços.