Hi/Здравейте:)
I see you made some correction on border. 3-4 years ago, Serbia released all boundaries as open data, so we used that to create all internal and some external boundaries (at least with countries also having open data boundaries). Question for you - do you maybe know if Bulgaria have some boundaries I can check and compare. Ideally, we can compare our boundaries and create boundary in OSM that both communities can agree upon. I see you mentioned source here, but I don't know how to see anything on it