This is a great question. You are correct, Polybius wrote in Greek in his work, the Histories.However, he wrote about Rome and perhaps most importantly, he is one of our earliest sources.
Polybius had ample interaction with and knowledge of the Romans. He was an Achaean statesman when Rome’s presence was a real force in the Greek world, and subsequently he was one of the thousand who was taken to Rome when the Romans began to suspect the loyalty of the Achaeans after their war with Perseus, the King of Macedonian in 167 BC. During this time (167-150), Polybius stayed in Rome and became associated with the Roman elite.
He also became the tutor of Scipio Aemilianus and subsequently accompanied him to Africa and Spain (150 BC?). He was also present at the destruction of Carthage (146 BC). In short, Polybius was an eyewitness of the second century and an insider of Roman elite culture. It is no wonder that the great Roman annalist, Livy followed him at times in his own work. Therefore, Polybius is one of our best sources for the Roman Republic, even if he wrote in Greek.