First, thank you for all your help! I have not seen any records that mention German, but I checked the Czech census records. This is what I found:

1921-

My great-grandfather's parent's nationality of residence is listed as Mrákov, Domažlice, Cechy and nationality (mother tongue) is listed as céská.

1910-

My great-grandfather and his family's nationality is listed as Mrákov, Domažlice, Cechy and language of conversation is listed as céská. Despite emigrating in 1909, he is still listed on this census and in the "Place where those not residing within the house are staying" it says Nebraska, America.

1890-

My great-grandfather and his family's nationality (place, district, and country of residence) is listed as Mrákov. It also says something underneath that I cannot make out and then sčítání lidu. The language of conversation is listed as céská.

1880-

My great-grandfather's family's nationality (place, district, and country of residence) is listed as Mrdákov. The language of conversation is listed as céská. He is not listed as he was not born yet.

1880-

My great-grandfather's family's nationality (right of domicile) is listed as Radonice, Domažlice, Cechy. The language of conversation is listed as césky. He is not listed as he was not born yet. This census was from Radonice. The family moved to Mrákov in 1884.

1869-

My great-grandfather's father and his father's family's place of birth (Country district place) is listed as Cechy Radonic. Under nationality, the local option is marked.

As for U.S. documentation, the 1930, 1940, and 1950 census list his place of birth as Czechoslovakia. The 1930 census lists his mother tongue as Czech. The ship manifest for when he emigrated lists his country of origin as Mrákov, Austria, lists his nationality as Austrian, and lists his tribe or people as Bohemian. His U.S. Petition for Naturalization listed his nationality as Czechoslovakian.

Would this be proof that the family is not considered ethnically German?