My great grandfather came from Austria-Hungary, the part that became yugoslavia and what is now croatia. He had a Czech name, however his mother tongue is unknown.

A DNA test on my grandmother confirmed that he was slavic, but which ethnicity is unknown to me.

Were there Czechs in A-H at the time, or could he be a mixture of different slavic ethnicities?

He immigrated about 10 years before WW1 if that helps.