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?
Miners join mining pools to produce a bigger hash rate and catch blocks faster doesn't man that they're centralizing Bitcoin. Bitcoin itself is still very much decentralized. The protocol hasn't changed a bit and it remains decentralized.
Like the other posters have said US overtook China after the ban. So they're leading now in total hash power. Achieving 51% hashrate not that can be used to attack the network and attempt changing the protocol isn't an easy thing at the level Bitcoin has grown to today, so there's little concern that the action of a mining pool can centralize Bitcoin.