A loan can be denominated in a currency other than what you directly pay for it with. That just means somebody is automatically converting it for you. Financially, it's exactly the same as using your Visa to buy something in a foreign currency; you're paying in Euros for a loan denominated in dollars, and you'll pay it off in dollars.

The loan can't be denominated in multiple currencies at once, as you seem to be thinking of, because that would make the loan amount undefined unless the two currencies were directly pegged to each other.

You can have a loan of 100 dollars. Or you can have a loan of .01 BitCoin. You can pay either off in dollars or BitCoin. How much a given pile of currency will pay off depends on exchange rates. But that doesn't affect the loan itself.

I somewhat sarcastically said nobody would denominate a loan in BitCoin, but not sarcastically at all, nobody would ever denominate a loan in "either this much of one currency or this much of another" because that is seriously a no-win situation for the entity loaning money. Obviously the loanee would pay it off in whatever is cheaper.