One is the equivalent of Lei in italian... you can be extremely polite by using it... however, it doesnt conjugate like he/she the same way as Lei does with lui and lei in italian.

We use 'one' in English when we are not referring to a specific person, but to any person. The reason it is not 'one says', is because it is a question with the auxiliary 'does'. In questions with the auxiliaries 'do/can/should/would' etc, the main verb is in base form. e.g. 'One TRIES one's best, but one CAN'T always GET what one WANTS' 'Where DOES one GO to get some peace and quiet in this place?

Here. I don't feel that I explained it correctly. Third person singular uses "says" you are correct , however this is an interrogative sentence and therefore it is "say". Check out this link "http://conjugation.com/verb/say" click on the "interrogative" section. It explains it all there. "do I say ? do you say ? does he/she/it say ? do we say ? do you say ? do they say?"

The reason there isn't an "s" after "one say" is because it is singular. How does "one" say? You see? One could also say "how do you say this?" or "how would one say this?"