Can Bitcoin ever be use as other country currency and it will still maintain it decentralized digital currency?
Only in case the country will be with the same name Bitcoin and with the same idea and principle of organization. And it is hard to imagine that some day some country like that would be created with quite another rules of life and quite another laws and so on... It may be in dreams only, at least in the period of our civilization. The key idea of Bitcoin is not to be under control of banking central systems, to be independent and decentralized.