I'm rating it 3 starts until it gets fixed: carvariations.meta and vehicles.meta don't work. Carvariations.meta prevents the lightbars from functioning (activating the siren makes all lights go out) and vehicles.meta makes it spawn with the default skin (usually), not this reskin. Moreover, it's not mentioned in the readme.txt where visualsettings.dat should be placed (and what carvariations, vehicles and visualsettings are needed for). I've made sure that everything was installed correctly. Other than that, the model/reskin itself works.
Just out of curiosity - why did you make a Polish skin if the description is neither in Polish nor even in English?

[ENG] Fix for lightbars and reskin: install carvariation.meta from the original model here https://www.gta5-mods.com/vehicles/2014-police-mercedes-sprinter and vehicles.meta from a comment on that page or try it without changing vehicles.meta first (just don't change it).
[PL] Jak naprawić koguty (nie świecą) i malowanie: zainstaluj carvariation.meta z oryginalnego modelu z powyższego linku, a co do malowania, jeśli samochód pojawia się z innym skinem, pobierz i zainstaluj vehicles.meta z komentarza na dole pod tamtym modem.

I'm not sure if visualsettings.meta is needed as well, but in the original Sprinter model it says where to install it (although you can also use it with the one from the original mod).

Ok, seems like carvariations.meta DOES WORK, but vehicle.meta doesn't. To be exact, when spawned, the car would appear with a random skin out of 3 (2 were British, I think), one of which was the Polish one. My bad.
@Drixize could you update the mod by exchanging your vehicles.meta with the one I mentioned above? I can't give you more stars until you fix it. Adding my comment to the description isn't the same as updating the files.

[ENG] 4,5 stars for the textures. It's not exactly the same as the real one, but I'm not judging the model itself, as the author isn't a modeler. Compared to real-life, the biggest difference is no (working) beacons on the sides and below the riot mesh (like the one above the doors, which are modeled but don't light up) and meshes on headlights. The real Polish vehicle doesn't even have lightbars. Less important imperfections are the dogs in the back which shouldn't be there, too high in size white stripes (which are silver-white IRL) on the vinyls, too high hood vinyl compared to the real one and the LEDs should be of a violet-blue shade (it might be possible to modify in carcols.xml):
https://img.czerwonesamochody.com/data/media/659/Z799_PRZOD_LEWY.jpg
As for making it completely perfect, this is how the vinyls look like zoomed in:
http://www.skierniewice.policja.gov.pl/dokumenty/zalaczniki/249/249-1950.jpg
Other than that, it's a great reskin.

[PL] 4,5 gwiazdki za tekstury. Nie wygląda idealnie jak prawdziwy, ale nie oceniam samego modelu, ponieważ autor nie jest grafikiem 3D. Przy porównaniu do zdjęć największą widoczną różnicą jest brak specyficznych kogutów na bokach i pod siatką zamiast lightbarów na dachu (jak te nad przednimi drzwiami, które niestety nie świecą) i siatek ochronnych na światła. Jeśli chodzi o mniejsze niedopatrzenia, psy (hehe, psy w samochodzie policji...) są niepotrzebne, białe paski na winylach nieproporcjonalne, winyl na masce jest za duży w porównaniu do prawdziwego, a LEDy powinne być fioletowo-niebieskie:
https://img.czerwonesamochody.com/data/media/659/Z799_PRZOD_LEWY.jpg
Tak powinny wyglądać tekstury, ale to już całkowity szczegół:
http://www.skierniewice.policja.gov.pl/dokumenty/zalaczniki/249/249-1950.jpg
Oprócz tego świetny reskin.