whatever you want. Class I&II would be a better description of the Serbian classifications, national/regional/motoways would be more generic. There is no standard yet in OSM (Germany has no network=, except BAB->Bundesautobahn=Federal motorway, which I think is a mess…a german acronym in an international project). Personally I prefer national/regional/motorways because it is a step forward to an OSM standard and people not familar with Serbian (or Poland or Japan…)specific classifications, will find such generic classes easier and will not need to find the classes for each country individually.