Your Polish word directs to
Denouncer (latin Delator)
Denunciation - an open public accusation or reporting a person or a group of persons to public authorities, often done anonymously for low political or personal motives, on which the informant has a personal interest or hopes to gain personal benefits.
Informer/Informant is closely related
An informant is a person who provides privileged information about a person or organization to an agency. The term is usually used within the law enforcement world, where they are officially known as confidential or criminal informants, and can often refer pejoratively to the supply of information without the consent of the other parties with the intent of malicious, personal or financial gain. However, the term is used in politics, industry and academia.
Informal synonym: Stool pigeon which I have only seen mentioned in relation to criminal circles.
Grass is an informal synonym to informer and is used mainly in Britain.
informal A police informer.
[perhaps related to the 19th-century rhyming slang grasshopper 'copper']
replying to a comment and another answer, I would not use the words grass or snitch for a
- Chinese that reports a neighbour for having Falun-Gong books on his bookshelf
- Ugandan that tells the police his male employer has a boyfriend
- Someone informing Gestapo that the priest is hiding Jews in the church.
I would more likely use them for a schoolboy that tells the teacher that one of his bullies has a reefer in his bag or the drug runner that sells out his supplier for leniency in court, which rhymes with the usage mentioned in the comment.
In my opinion snitch/tattletale are synonyms for petty telling on classmates and grass one step up in criminal circles and none of them related to reporting people to the government to get even with them or for personal gain like taking over their house, although such motives can apply to snitches, grasses et al