How does the Croatian system of flour categorization correspond to the systems of Western Europe and USA?
Flour types are quite different in various countries, but yes, all flours are basically made from crushing grains (wheat in this case).

A grain mainly consists of three parts:


 Source: http://www.californiawheat.org/industry/diagram-of-wheat-kernel/ 


The bran.
The outer layer of the grain.
The endosperm.
The white inner part that we tend to associate with flour.
The germ.
The sprouting section. Often removed in flours for longer shelf life.


So when milling grains, the parts of the grain are separated, for whiter flour (that's the lower numbered types, the number actually giving the mineral content and indirectly indicating the fiber content.), the bran and germ are mostly removed, "brown" and "black" flour (middle to high number range) increasing amounts of the bran are kept,resulting in a higher fiber content.

Whole grain is - as the name implies - made from the whole kernel (but quite close to "black" / 1600).

As you noted, the "numbers" (= mineral content) typically sold in stores are not consistent in European countries, but the pattern is always the same. (In France, the numbers seem different, but are basically the numbers you know divided by 10.) If your recipe calls for a specfic "number", you can "mix" "lower numbered" and "higher numbered" flours to get the desired strength.

For simplicity the lighter types are sometimes labeled according to their use: Cake flour / all purpose flour / bread flour (-> increasing "numbers").

The US has a slightly different system, here flours are characterized by their protein content and labeled according to use. if I remember correctly there was some kind of "substitute chart" here, but I can't find it at the moment.