For a long time, Serbia (I am actually thinking of Yugoslavia, as I don’t know is this is current information I am giving) still used the German MG42 (and perhaps some MG34s as well - not sure on that one) in its original 7.9 x 57 mm version. Don’t know if Serbia ia still using them, though.
If you get into pistol, then some of the Eastern countries are still using the 9 x 18 mm Makarov (also in SMGs), and with the number produced, we are not likely to see its disappearance from the scene in our lifetimes, or certainly in mine. It has been “officially replaced” in several countries, although pistols and SMGs in that caliber are still probably in use, at least with Militia, with the 9 mm Para cartridge. Russian has announced the replacement of the Makarov Pistol, officially, three times now with three different pistols. Wonder what the fourth one will be. :-)