There are at least two Qlik Sense extension available on Qlik Branch that could be used:
Then you could use the variable-value that is selected to filter the right currency to get the exchange rate that is correct.
You could also do it natively in Qlik Sense without an extension but not as elegantly:
Let the users have the title "CTRL+Click to select a single currency". The translation table could be a data island table or and then use Only() or Min() or Max() together with Set Analysis/Set Expression to get the right value.