Get the default currency for a country:
<?php
$formatter = new NumberFormatter('de_DE', NumberFormatter::CURRENCY);
echo $formatter->getTextAttribute(NumberFormatter::CURRENCY_CODE);
$formatter = new NumberFormatter('en_US', NumberFormatter::CURRENCY);
echo $formatter->getTextAttribute(NumberFormatter::CURRENCY_CODE);
$formatter = new NumberFormatter('ja_JP', NumberFormatter::CURRENCY);
echo $formatter->getTextAttribute(NumberFormatter::CURRENCY_CODE);
?>