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It is easier for El Salvador to develop bitcoin as a legal means of payment because they do not have a national currency in their country, thus allowing bitcoin to become a legal means of transaction in that country. But if we talk about other countries, I am a bit pessimistic that what El Salvador can do smoothly, because other countries have official currencies that are regulated by regulations and rules within them. Other countries have more potential to use bitcoin as an investment asset, because this is much more likely to be implemented as is done by most countries currently because legal payments are still quite difficult to develop.
It may be true, countries that have their own currencies tend to have more difficulty adopting bitcoin as a legal currency (based on law) despite the fact that bitcoin is legally used as a means of payment in several countries that have their own currencies. El Salvador is certainly different from most other countries, but that doesn't mean other countries can't adopt bitcoin like them even though they have their own fiat currency.
I don't have valid data on how many countries where bitcoin can be used as legal tender, but it is true that bitcoin tends to be used more as an investment and trading asset than as a real use case. Indonesia is one of the countries that does not legalize bitcoin as a means of payment, but investment and trading are legal.