The German (and other) naming style of multiple forenames being used interchangeably frequently carried over for up to several generations post emigration / immigration.
So, just because a German family has lived outside Germany for a couple of generations, it should not be assumed that they suddenly have a "first name" and a "middle name" because they were born in, say, the United States, or Australia, etc.
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(I'm Australian, not of German descent (although I have German relatives) and I recently checked my own birth certificate, as well as those of my children, and other family members, and it is quite clear to me that on those official Australian documents issued by New South Wales, Western Australia, Queensland, and Victoria, that we all have a name, and a last name. There is nothing on any of the official documents to say any of us has a "middle name".)