Some remarks about localization an app in iTunes:

1. The country associated with the Apple ID has to be one of the countries the app is published in to be able to find and install the app. So this has nothing to to with Settings / General / International / Region Format!
2. You can change your country only by also entering valid payment info, so there's no way to simple switching between countries.
3. If you have localized an app, the language that matches the Settings / General / International / Language is displayed. So if you are from the Netherlands and have English as language, you'll see the English localized descriptions of the app.
4. If you changed your language, but still see the old localization in the app, logout and login in Settings / iTunes & App Store / Edit your Apple ID.
5. Localized information, like app name,  description and keywords can only be changed with a new release!