Exactly. The intl methods can help you with that.
You need to use Locale::getDisplayRegion
Object oriented style:
static string Locale::getDisplayRegion ( string $locale [, string $in_locale ] )
Procedural style:
string locale_get_display_region ( string $locale [, string $in_locale ] )
Returns an appropriately localized display name for region of the input locale. If is NULL then the default locale is used.
So, in your case:
php > echo Locale::getDisplayRegion('nl_NL', 'nl_NL');
Nederland
php > echo Locale::getDisplayRegion('en_GB', 'nl_NL');
Verenigd Koninkrijk
php > echo Locale::getDisplayRegion('nl_NL', 'en_GB');
Netherlands
php > echo Locale::getDisplayRegion('en_GB', 'en_GB');
United Kingdom
Remember that 'GB' is used for the United Kingdom instead of 'UK'