When going to buy Gems, it asks for the billing country so it can use the appropriate currency. I’m from New Zealand, and after entering that it just comes up with the prices $10, $20, $35, $50. It doesn’t say anywhere that it’s in USD, all it has is the dollar sign.
This is quite ambiguous (one could quite easily assume it was in NZD, seeing as we entered NZ and that it would choose the appropriate currency – I thought there’d be a fairly high change it was meaning USD, but others may not) and so it should show the currency being used.