1. All of a sudden
Something has occurred in a very short period of time. There has been a shift in thinking. In the dialogue, their desire for Japanese food increases as Sergio talks about the different kinds of Japanese dishes. When the desire arises it comes very fast. It comes suddenly.
2. That's too bad because...
This is another way to say, "It is unfortunate because..." In this example, a statement will follow to explain just why it is unfortunate. It is a common way to speak between friends. If the situation is more formal in nature, then you would more likely use the term "unfortunate". Remember that colloquial speech is used by people who are familiar with each other.
3.Wait a minute
You could also say, "Hold onto your horses." They would have used this when cowboys rode horses and not bicycles. You are telling someone to slow down their decision making process long enough so as to get all of the details. Kind of like saying, "Wait until I finish (about a minute) before you make a hasty decision."
4. ... sound very good
In this case, since we are talking about food, the sound refers to the taste of the food. I know that they have yet to taste anything but when food is described in the right way it creates an imaginary flavor in your mouth. That flavor comes from the words used to describe it. That flavor comes from the sound. So if it sounds very good then you will want to have some.
5. It's settled
Everyone is in agreement. The discussion, argument, or negotiation is settled. In the dialogue, the discussion is about selecting the best tasting dessert. Once they have agreed then one can say, "It's settled." In other words, they are in agreement and have made the same choice.