Such terminology is closely tied to the peculiarities of the legal system of a particular country, and there is therefore no straightforward way of translating a term like this into the language of a country with a different legal system. If one is speaking to an audience whose country happens to have something very similar in its legal system, one may use the term that is used for it in that country as the translation. That will, however, rarely be the case.
If the audience is from a country whose legal system does not have anything sufficiently similar, then one has to determine how important the precise legal status of the officers in question is to the point of the communication. If it isn't, one may use some term that is not exactly accurate (e.g. police officer in this case), as long as the inaccuracies do not interfere with the point one is trying to convey. On the other hand, if the point hinges on the precise legal status of these officers, then the only solution is to explain it, by saying something like 'In our country, we have a class of law-enforcement officers who have powers A, B, and C, but not D, E, and F. They are appointed by X, and supervised by Y. In our language, they are called . . . , which literally means . . . '.
It should be noted that the problem here is not, strictly speaking, a translation problem. It is not due to the fact that Polish and English are different languages, but to the fact that Poland is a different country from the country of the audience. Problems of the same kind arise when one is speaking of matters related to, say, Australian law to a U.S. audience, or the other way round.