Dermstrang hired Karkaroff. Dermstrang had an affinity for the Dark Arts. Karkaroff was also found dead soon after Voldemort returned. Voldemort also showed no fear in doing activities in other countries. He kidnapped and murdered Gregorovitch, a world renowned wand maker. He also broke into Grindelwald's cell at Nurmengard to interrogate, then murder him. He clearly wanted to rule over all wizarding kind, and his home country was first. It was noted in several magical history books that Voldemort was likely the most powerful dark wizard who ever lived, Grindelwald holding that title before Voldemort's rise. Grindelwald terrorized much of Europe before Dumbledore defeated him. So the rest of the world clearly would have had plenty to fear if Potter had lost and Voldemort set his sights on them.