The dollar will keep devaluing long term, not just because of the inflation and the printer go brrr, but also because the most adequate world economies are giving up on it as the reserve currency. Euro, yuan, even the Russian ruble if you feel adventurous, and of course gold.
The question is, if you hold foreign currency as an American, how can you hold it that it would retain its value (and thus its only utility) but also be out of reach of the American Gov confiscation when they decide to make it illegal to hold foreign currency for the non-rich?
Thanks to FATCA, Americans outside of HNWs de facto cannot hold assets in foreign banks, and it all has to be reported (banks will enforce this thanks to US' control of the SWIFT system).
If you have foreign currency in an online account with a US institution, a couple clicks by US Gov and it's gone. But you can quickly trade it. But physical currency is no better (though no counterparty risk), since you need to physically exchange it. Americans probably don't even know anything about foreign currencies, and they aren't likely to accept it in place of USD without knowing a real time index of its value (which could be faked or unavailable due to regulation). Foreign currencies, afaik, aren't legal tender in the US.
Better to buy things for immediate survival, food & water, barter items, things that don't require a functional society to be able to function & aid you (ie, bicycle, bike trailer/racks & parts rather than car).