If the price of in-game items is displayed in a foreign currency, this issue may occur due to various reasons (e.g. temporary system error).
Hence, please attempt the following steps to resolve the issue;
1) Close any applications other than Dark and Darker Mobile
2) Check and update your OS version
3) Go to your mobile device's Settings > Applications > Select Dark and Darker Mobile > Storage > Clear Data and Cache
4) If your account is linked to SNS, uninstall and reinstall the application
5) Verify the country information of your payment account (Google Play Store / Apple App Store)
If the issue still persists after attempting the steps above, please contact the customer support for further assistance.