The Romanian Longhorn Dragon Reservation is likely located in the mountains of that country. The reservation, founded by Harvey Ridgebit in the mid-1900s, is where Charlie Weasley works, and where his parents once visited him. It doesn’t seem to just be home to Romanian Longhorns, as Charlie’s crew brought at least one dragon for the Triwizard Tournament in 1994, and none of the dragons were Longhorns (GF19). We also know that Norbert, a Norwegian Ridgeback, was sent there as a baby when Hagrid realized he could no longer handle raising her (PS14).
The main mountain ranges of Romania are the Eastern and Southern Carpathian Mountains. The dragon reservation is most likely located in one of those ranges.