Josip Kriaj was Slovenian (or Slovene). Kriaj is common surname in Slovenia and his mothers maiden name Vidmar is also typical Slovenian surname.
The history of his birthplace, the village of Kopriva near Seana, may help to understand all this. In 1911 when Kriaj was born, Kopriva was in Austria-Hungary. After WWI it became part of Italy. It was at that time that Slovenian names were Italianized and Josip became Giuseppe. After WWII Kopriva became part of Yugoslavia. Today, Kopriva is in Slovenia.
From following two sites:
Information on http://aces.safarikovi.eu/victories/yugoslavia-sp.html
is somewhat missleading.