I guess they will learn from market fluctuations like how we did since 2013.
People from El Salvador who buy bitcoins and spend it for their day to day life's need will not mind about the fluctuations of bitcoins but people who get into bitcoin just for the purpose of short term holding might be facing problems of losing their hard earned money value due to bitcoin fluctuations.
One of the probable solution here for El Salvador citizens must be, if they spend for bitcoins for the purpose of using it as a legal tender then they must spend bitcoins as early as possible; it means do not buy bitcoins for month long expenses just buy bitcoins for one or two days' expenses hence short holding may not hurt much. This way they may get chances to escape from wild fluctuations of bitcoins.
They already know that the price would go up and down, they even said so themselves before they got into this. It is obvious that El Salvador got into it fully knowing that they were both a bit late to this current run (from 4k last year to 40k+ this year) but they also knew that it would be a great long term deal for them.
This is why I believe that they are fully proud of what they are doing right now. Obviously there are protesters and all that, obviously there will be people who are against it, however that doesn't change the fact that people who supported this movement there would still be supporting and wouldn't be changing their minds just because we had a little correction. It will not change and we will be doing a lot better so they realized long time ago that they will be doing fine in the long term and do not care about these short term movements.