Internationally banks simply have to be able to acquire sufficient foreign currency holdings. Which is trivially easy because every competent reserve bank on the planet has enormous foreign currency holdings.

But this is all irrelevant to Bitcoin - which is marketed by its advocates a consumer currency, not an international system of exchange between institutions. Institutions have no need nor desire for such a thing - it is a saturated marketplace, with literally thousands of avenues of exchange, of which Bitcoin is a particularly poor one.

Which circles back to my original point: foreign currency transactions are very simple for anyone ranging from consumers to medium or large size businesses, barring tax issues (like not paying a lot of it). Bitcoin does not solve a problem not already solved for centuries by the banking system - this is literally the thing that got it started way back with the Knights Templar.