Rob, what they used (and continue to use) in German is multiple given names. Sebastian was _not_ Bach's middle name. It was a second given name: just as much his name as Johann, not in a separate category like the American concept. He was free to use either or both of Johann and Sebastian. (Given that both of his brothers and several of his sons were also Johann-Somethings, I'm sure he didn't go by just Johann.)
In English-speaking countries, there's a separate category of "middle name". (They're not part of people's official names, according to the U.S. Social Security Administration.) In the rest of the world, everything is either a family name (surname), or a given name. Laws and customs vary in terms of how many of each you can have and where/who you get them from, but they all agree that the elements within a category are of equal "weight" or importance.