"Are you going to the city?"

Translation: A városba mész?

[faltayje]

"you" could be plural. If it is plural, is the word "mentek" correct?

[vvsey]

Yes!

[Qnan]

What about the polite form, "Mégy a városba?"

[RyagonIV]

It's fine, too, but the correct form is megy, with a short e.
Okay, to be fair, mégy is also a correct form, but vastly less frequently used. It's the same conjugational form as mész.