Why does Billy become confused when the skiers arrive to rescue him after the plane crash?

Billy got confused because the skiers were speaking German. He didn't realize where he was and believed they had something to do with WWII.

The people who first got to the crash scene were young Austrian ski instructors from the famous ski resort below. They spoke to each other in German as they went from body to body. They wore black wind masks with two holes for their eyes and a red topknot. They looked like golliwogs, like white people pretending to be black for the laughs they could get.

Billy had a fractured skull, but he was still conscious. He didn't know where he was. His lips were working, and one of the golliwogs put his ear close to them to hear what might be his dying words.

Billy thought the golliwog had something to do with the Second World War, and he whispered to him his address: 'Schlachthöf-funf.'