To help improve game monetization, Facebook launched a new API called "Preferred Currency API," that gives you access to the user's preferred currency and the conversion rate to Credits.
"This lets you show prices in the user's preferred currency in your in-game stores," Facebook said.
Above screen shot shows Castleville's in-game store and payment dialog show items priced in EUR, this user's preferred currency.
"Applications can display the prices for in-app purchases in a user's preferred currency, rather than in credits. To be clear, all items will still be priced in credits, the currency that the Facebook payment system is based on; however, you can display prices to your users in the equivalent amount of their preferred currency, and invoke Facebook's payment dialog in a way that will cause it to do the same," Facebook explained.
"For example, a user visits your store to buy an item priced at $10 (100 Credits). You can access the preferred currency -- EUR for this user -- and the current conversion rate from Credits to EUR via the API described below. Instead of displaying price in USD, you can now show the item price as €8.12 (100 credits * 0.0812 conversion rate). When you invoke the pay dialog, use the parameter "oscif=true", and the user will see €8.12 as the price of the item in the pay dialog as well," explains Colin Creitz.
To access this data, issue an HTTP GET to /USER_ID?fields=currency with a user access token for your app.
For more details on how to implement this in your app, check out this documentation.