Wizards is in the USA, though.
BTW, with a 5% interest, leaving your money in the bank for 1 year will approximately multiply it by a factor of more than 50 millions. One year from now, I could very well have around 27105920788 money. Assuming that I stop posting. Which is a bit like assuming that the sun will not rise.
Even with a 2% interest, though, your money is multiplied by more than 1300. Much less, but still a helluva lot. But it would mean no monies for healing unless you post a lot or fight very little.
Don't you have to love exponentials? Interest is definitely going to give a big advantage to "old players". I think that a cap to what the bank can hold could be appropriate. Something like 10000. It would kill the exponential nature of interest without impacting poor people. Is it already in place?