But, the government of some countries are against Bitcoin and some are ready to adopt Bitcoin. So what would effect on Bitcoin if any government approved Bitcoin as a legal tender?
There is virtually no point in adopting centralized cryptocurrencies at least in my opinion, simply because of the fact that it's essentially the same as fiat currency - it's not backed by anything, its supply can be debased, and it's just totally controlled by a single entity.
Your last question didn't make much sense. I'll still try answering it, though. There is going to be no effect on bitcoin itself if it ever becomes legal tender in a particular country, but it will have an effect on those who live in that particular jurisdiction, as the legal definitions of bitcoin may change.
Got your point, in the first place why do we need centralization? Cryptocurrency is made to free us from any third trusted party thus its ntaure is to be decentralized. With or without government intervention, Bitcoin will continue to go on. When government intervene on tge system, we already have no full control over our funds. Centralization may just cause complicated system, so I don't see the for having centralized system. To maximize Crypto feature, don't use banks, closed source wallet or anything that will control your fund. Choose an open source storage lIke HODLER wallet, this makes you have full control over your coin.