Certain countries using Bitcoin won't really directly make Bitcoin more or less decentralized.

With that said, Bitcoin as a country's currency? Very very unlikely especially for the bigger more established countries. Bitcoin as one of the accepted payment currencies along with the country's local currency? Sure.

Decentralization has nothing to do with that.

Bitcoin is decentralized because there is no single point of failure. You cannot shut down the network attacking one person, or one company.
Bitcoin is decentralized because nobody owns the code. There is no single person who says what miners or developers should do.
Bitcoin is decentralized because there are thousands of full nodes. Anyone can be a full node.
And so on.

if a country decides to use it as a national currency, none of those things will change.
However, bitcoin the **only** currency was never bitcoin goal. Bitcoin is an alternative to the fiat system, but they are not competing. Both can coexist.

bitcoin to accept payments is possible, but to be a currency in a country probably not, and getting bitcoin is currently a little difficult

Bitcoin at its current state being used as an official currency? Not going to happen. Anyone that says that it is possible is actually being delusional. Look at all the current problems bitcoin has. At this current stage, people aren't using it as a currency for regular transaction, let alone a nation. The fee and waiting time is what making people not to use it as a currency. Hence people find it more suitable to use it as a form of investment due to its store of value.
A country may eventually make their own centralized crypto currencies though. There is no denying to that.

If it is used as a means of payment in another country (buying paying for something with bitcoin), I don't think so. Because several countries have banned the use of cryptocurrency as currency. Also, not many shops or merchants accept bitcoin as a means of payment.

However, if used to exchange FIAT, of course you can. I think every country has an exchange that will convert bitcoin to their national currency such as USD, KRW, IDR and others.

This right here.
Legal tenders are currencies accepted by that country or jurisdiction as the means of exchange and one of its main qualities is the ability of the government to regulate it; control money printing and operate the banking system. This is not possible with Bitcoin, the country would not control it as it is not centralized and they cannot account for it when it is used for trades in other countries. It's sort of like accepting to use something virtually every other nation is using anyways.

While the idea sounds interesting, it is not practical, at least not in the way current societies are run.

Correct me if I'm wrong but when countries do this they do not directly adopt fiats of other nations but rather create their currency system which ideally mirrors that of the other country. So their fiat value would depend on the value of the original currency, such as what we have with the Sudanese Pounds. However, as with the SDG, internal economic crisis could influence the value of the currency and it could become devalued against its parent currency.

See, it's impossible for  one country to manage a btc or inherit it as a source of everything and if it happens btc is in control of such particular country and is no longer a decentralized currency, and it's obvious that many countries are using bitcoin as a means of payment due to how influential bitcoin it, but that really means that such country have a control over it's regulations.

You probably wanted to ask if Bitcoin could be used as a national currency in some country.
I don't think that is possible because Bitcoin is not centralised currency and I don't think that any country would like to have economy that is dependent on volatile, limited and unregulated currency. So, no, that will not happen
Besides Bitcoin is not meant to be dedicated just to one country.

Yes I believe, in countries which are underbanked or underdeveloped. Bitcoin would surely fix problems such as counterfeit money. It is never going to be a country's main currency though.