This is a very broad question to answer and you can't pinpoint a certain empire or kingdom. To the very least it would be an anachronism. But for the sake putting some context - Saints Cyril and Methodius are credited with the creation of the Glagolitic Alphabet for Great Moravia where it was used to translate the Bible and for government and religious documents and books between 863 and 885. Afterwards this was prohibited by Pope Stephen V in favour of Latin. Students of the two apostles, who were expelled from Great Moravia in 886, brought the Glagolitic alphabet to the First Bulgarian Empire. Later on the the Cyrillic script was developed early on at the Preslav Literary School where it superseded Glagolitic. In it's early form it is know as Old Church Slavonic and spread to other South-Eastern, Central, and Eastern European Slavic territories, most notably Croatia, Serbia, Bohemia, Lesser Poland, and principalities of the Kievan Rus' while retaining characteristically South Slavic linguistic features. Many prayers, liturgies and books were translated to Old Church Slavonic in the First Bulgarian Empire and many were brought to present day Russia where Old Church Slavonic maintained prestigious status for centuries. In a way it was, to the East Slavs something like the Latin for Western Europe.