Its hard to know specifically what being legalised means in El Salvador since the $ (US) is their national currency Making BTC legal I suspect doesnt mean someone has to use it, or accept it or anything else I suspect it means that they will treat it as having dual currencies and as such you may get either or both. For example in the UK BTC is legal tender but its not recognised. So if you buy something in BTC the shop would have to convert it back in to FIAT (GBP) in order to say what price you had paid and deal with taxes and profits and statutory bodies /filings accordingly. What is possible is that El Salvador will allow say payment in $ and BTC and when people do taxes and make payments and shops pay, make payments, declare profits etc they can do in either or both depending what the recipient is willing to accept. From what I can tell its not replacing the dollar but will sit with equal recognition.