Which preposition to use with slavonians

in Occurrences 1%

879. Methodius forbidden by the Pope to perform the services of the Church for the Slavonians in their own language.

of Occurrences 1%

The Bulgarians of the Danube, the Moravians, and the Slavonians of Illyria had been already enlightened by holy baptism about the middle of the ninth century, during the reign of the Greek emperor Michael and the patriarchate of the illustrious Photius.

Which preposition to use with  slavonians