Something that I find very important is to get and hold new mapper. This means marketing in GPS forums and someone from Serbia should welcome new mappers in Serbia in their own language and point to the forum. If they socialise, they will stay longer and contribute more. Just a week ago someone new joined in Kragujevac....in fact every day someone new joins and adds two or three edits and leaves OSM. But the guy in Kragujevac has added a significant amount of data so it might be worth to say welcome to such people.