Click the menu button
Click "Settings"
Click "Language" to change the language settings
Select Korean (The language on the top)
You may be required to accept an agreement after changing the language. Select the red box to continue.
Click Refill coin (indicated by the coin icon)
If you want to purchase points bundles click the button on the right. Otherwise select the regular coin bundle you'd like.
Click the dropdown arrow to see additional options
Click "Payment methods"
It's not shown here, but many phones also have an option to bill your mobile carrier. The payment options available vary by country
Note: if this is what you see after selecting the top payment option, you are not on the right app. You must use the Google Play Store app