I heard that most of the miners are located in China. Thus we can say that most hash power is controlled by some regions. Can we still call Bitcoin decentralized if its backbone infrastructure is so centralized?
Decentralization of Bitcoin is guaranteed by nodes, not miners. Miners only package transactions and must serve the nodes. If miners try to do something that would anger people who hold and use Bitcoin, the community can always change the hash algorithm. Then miners would be in a tough spot, because the equipment they invested billions of dollars in would become worthless. Thats why miners have to listen to Bitcoiners, users, and the people running nodes.
Thats why its important for as many people as possible to run their own node, as it guarantees decentralization and ensures that no one can manipulate the network on their own.