You need to understand that under communism commodity production will be abolished, things will not be produced to realize a profit thru exchange like it is under capitalism, they will be produced with the sole function of satisfying human needs.

So currency will become worthless.

Now keep in mind that is the last stage of historical materialism (communism), and there will be transitory stages where you will still have commodity production, currency, hierarchical division of labour, etc (socialism), but yeah for socialism to be able to transition into communism you need some preconditions like internationalism, creating a superabundance via ephemeralization of production (thanks to technological progress and automation), etc.

Regarding the internationalist point, if the world-system remains mainly capitalist socialist experiments are doomed to fail or to stagnate, like history showed us, because the world is so interconnected that they necessarily need to play the capitalist market game and deal with trade, resource competition, geopolitics, embargos, etc.