You'd have to refine your question. Bitcoin is already a national currency.
Pontinha recognized it as a national currency back in 2015 (they're very small, located off the shore of Portugal). I don't think that some big country would declare Bitcoin as their national currency and especially not in 2016. The price is not stable enough to act as such.
I wonder how it went in Pontinha and how are the banks located in that place.
2026 may be different from what it is now and if government sees what good can bitcoin do. And since bitcoin can be traced, this will probably help corruption in the government. Bitcoin may not be national currency for each country but probably chose any altcoin.