"Polisi memakai topi hitam."

Translation: The police wear black hats.

[Kleoboule]

The policeman wears a black hat

[volumniax]

I would suggest: "The police wear black hats" or "Police officers wear black hats" if this is a general statement about the police service. If it's about an individual officer, "The police officer wears a black hat." (With some local exceptions, "police" usually refers to a police service, not to an individual police officer. At least in North America!)

[IanWitham1]

But Indonesian is not a North American language.

Indonesian has plurals but Indonesians usually use the same word for the singular and the plural. "Polisi" can be a policeman, a policewoman, or the whole police department.

[BorblaDr.S]

Thank you to make it ckear.

[svLjZvvx]

Should be either 'The policeman wears a black hat' or 'Police wear black hats'.

[KelpyPony]

"The police wear black hats" seems to have been added. Nice work everyone!