Kingdom of Great Moravia
That was more present day Slovakia/Czechy/parts of Germany/Hungary etc... not Poland except for a small part of Silesia which would've been occupied by the tribe known as Slezanie. Idk how some posters got the idea of Moravia as ancient Poland... The only Moravian tribe that would've existed around 966 would be a small portion of southern Poland.
Even though Poland as it's known was Christianized in 966 there were many tribes who remained pagan for hundreds of years. Was the 10th century and beyond where most people were illiterate, poor as hell, and were happy just to have some meat once in a while. The top 10% nobility (which was a larger group than in other European countries where it was usually only the top 1% of society) meaning szlaczhta which generally took pretty well to Christianity and Rome's authority in the decades and centuries following 966 save for a few exceptions
Some info on pre-966 Polish lands, tribes, languages, etc.
en.wikipedia.org/wiki/Lechites - Polish is known as 'Lechtic' language
en.wikipedia.org/wiki/Polish_tribes would include a small section of Moravians as one of the many tribes which were turned into a confederacy by the Piasts, including East German Slavic tribes - Oborites - (polabian slavs) from which several towns in Poland are named - Oborniki around Poznan, Oborniki Slaskie near Wroclaw, etc.
en.wikipedia.org/wiki/Polans_(western) - In the 9th century the Polans united several West Slavic groups to the north of Great Moravia. The union led by the Piast dynasty developed into the Kingdom of Poland, whose name derives from that of the Polans.
Most of the info that we have from pre-966 Poland comes from Charlemagne's conquering of the Veleti tribes and before that some documents from Roman historians and geographers like Ptolemy.
Still Gniezno, inhabited by a tribe called Polanie or not.
Gniezno was always thought to be, according to the legends and history of Mieszko I, to be the founding place of Poland.
If any Cyrillic exchanges occurred it would've been most likely with the eastern Polans who had the most contact with Kievan Rus