What if the currency exchange rate has changed significantly over the time of the loan?

No it is not halal. What was borrowed must be returned without any increase or decrease. If you want to consider purchasing power, then I would say for example, you are from Iran and you want to borrow some money from a person in the UK, then borrow gold from and then return the same amount of gold. In between, you may sell the gold for Iranian Riyal and use the Riyal for what you intended to use it for. Or if you borrow in Jordanian dinars and repay the same amount after twenty years and realise that the Dinars have decreased in value, then it not your fault. Someone else is stealing from both the lender and the borrower. The borrower cannot be forced to pay above the borrowed amount just because someone else is stealing the value form both of them.