Hi Bekaw,
Me and my boyfriend went through that procedure few months ago and it wasn't too difficult... My boyfriend is South African and he applied through their embassy for that Single Status Certificate. I am not really sure what are the options in different countries but I think the whole point is that somewhere on the paper you get from any official authority of your country it is clearly stated that you are single/not married. And of course whatever document you get you will have to get it translated into Serbian. For example here in Serbia valid document for such purposes (i.e. getting married) is a birth certificate not older than 6 months since you can see from it one's marital status... So, to conclude, its only important that they can see that you are not married from whatever document you may have... and I guess the name of certificate doesn't really matter. Al least I've got that impression.
Anyway, I hope I helped a bit
Bests,
Lj