~snip~
Yes, there is a difference, and not only currency can be a legal tender, but in this case Bitcoin is used in El Salvador as a currency if we look at most definitions of this word. Because generally currency is money used at some territory what is authorized by law at this territory. So Bitcoin is a currency of El Salvador.
Here's the definition of legal tender from the Bank of England:
Legal tender has a narrow technical meaning which has no use in everyday life. It means that if you offer to fully pay off a debt to someone in legal tender, they can’t sue you for failing to repay.
Whereas currency is money:
Money in any form when in actual use as a medium of exchange, especially circulating paper money.
So you can pay with any form of money, say bitcoin, anywhere in the world in which the other party accepts it, but only in El Salvador and Central African Republic bitcoin has to be accepted for any debt.