¿Cómo se dice en inglés? "Alguien quiere un helado"
¿Alguien quiere un helado? - Does anyone want ice cream?
Alguien quiere un helado - Someone wants an ice cream.
If it was at home after dinner one would most likely say "anyone want ice cream'? because there is probably no choice or different ice cream , but if you were out with friends it would be , " anyone want an ice cream" then you have multiple choices. And the "an ice cream " refers to one of any type you desire.
In UK/Irish usage at least, assuming it's a question, a less grammatically correct, but more colloquial, option would be:
"Anyone for an ice-cream?
There is also: "Anyone fancy an ice-cream?"