I would add that in English, "It's said, that..." and "They say, that..." are more or less interchangeable when quoting a traditional proverb or "scientific wisdom"-- but "they say..." sounds more natural when you're quoting gossip:
It's said/they say that "Too many cooks spoil the broth." (~ У семи нянек дитя без глаза -- "With seven nannies, no one keeps an eye on the baby.")
It's said/they say that birds evolved from two-legged carnivorous dinosaurs resembling a miniature T. rex.
BUT
They say that Tom Cruise is secretly gay! ("It's said..." -- это не ошибка, а тут звучит чем-то не идеально, ИМХО.)