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?
What do you mean be backbone structure?
Bitcoin protocol is decentralized and the action of miners doesn't in any way alter the protocol. Miners only work is to confirm transactions into a block and store in the Bitcoin Blockchain thereby securing the network.
Bitcoin decentralization is at the protocol level and not on the network layer where miners actions exists.