My experience is that people from Yugoslavia/the former Yugoslavia born around 1960 or later all speak very good English. I've worked with or collaborated with people from Serbia, Macedonia, and Slovenia, and known folks from Yugoslavia before it broke up, and to a person, their English was all very good. (This is in no way minimizing Ms. Muhl's accomplishment; I realize what a pain English is, especially the idiomatic part.)
[tangent warning] The best ESL speakers are from the small Western European countries (Denmark, Sweden, Netherlands, Finland). A close Danish friend explained to me that there are not that many Danish speakers (maybe 7-8 million) in the world, so all Danes are fluent in either English or German, often both. That would also make sense for the folks in the former Yugoslavia (and Bulgaria, Albania, etc.), except that for those born in 1960, they were still behind the Iron Curtain till they were 30, so Russian was likely their secondary fluent language, and I would think they were discouraged from learning English. But evidently not. [/tangent]