Using google translate
how can you say in English - what do you say? / what's new?
what do you say ? You would use this when asking what a particular word or phrase is appropriate for a situation.
what's new - this could be a greeting when people meet. Usually people who know each other and have not met for a period. But it also could be others as well.
I hope that the English below, translates to the Italian below
what are you doing these days ? cosa stai facendo in questi giorni?
This phrase would not be a greeting, but would be during a conversation between people who know each other but have been apart for some time - usually a significant time, years perhaps.
The people are said to be "catching up" with what each has been doing during the period.