Learn Spanish (Latin America): Cuentan que la bruja no duerme nunca.

There is no specific "they" here. This is an impersonal sentence. By using the 3rd-person plural (cuentan) without a subject pronoun, it indicates that "people in general say" or "the story goes." Adding ellos would completely change the meaning, making it sound like a specific, known group of people is telling the story.

Why use cuentan instead of dicen for "they say"?

While dicen literally translates to "they say," cuentan comes from the verb contar (to tell or relate a story). Using cuentan que... gives the sentence a storytelling flavor, translating closer to "the story goes that..." or "rumor has it that..." which fits perfectly with a sentence about a witch.

Why is it spelled cuentan and not contan?

The verb contar (to tell/count) is a stem-changing verb. In the present tense, the o in the root changes to ue for most forms, including the "they" form. That is why it becomes cuentan.

Why does the sentence use both no and nunca (no duerme nunca)?