Polish is one of the Indo-European languages. It is from the West Slavonic group which also includes Czech and Slovak. In the early Middle Ages proto Slavonic tribes moved away from their ancestral lands and settled throughout central, southern and eastern Europe. This expansion diversified the language of the tribes but, even to this day Poles, Slovaks and Czechs can communicate with each other without using an interpreter.
The continued development of the Polish language was closely tied to the early beginnings of the Polish state during the 10th Century. Diverse but linguistically similar tribes in the Orda and Vistula river basins were united by Mieszko I, who also went on to be baptized in 966. The introduction of the church also heralded the arrival of Latin, which at the time was the Lingua Franca of Europe. The introduction of Latin and the subsequent adoption of the Latin alphabet, made writing Polish, which had till that point only been a spoken language, possible. Early manuscripts were the province of the clergy but, Slavonic names and insertions were occasionally used. The earliest surviving example of this kind of text was written in 990-992, and it’s known as the Dagome judex, in which Mieszko I subordinated his newly formed Polish state to the Pope, and described his lands and the new cities Cracow and Gniezno.