Should just 'police' be a valid translation?
Yes. Police is a valid translation now.
what about cop? is there any differences between police, policeman, and cop?
'Police' can be plural, 'policeman' means a man who works in a police force and 'cop' is a slang term to mean 'policeman/policewoman', you wouldn't use it in a formal situation or to address a policeman/policewoman.
What's wrong with the word "policeman" ?
Polisi cowok terjemahan policeman.
Too long word, I guess