Magento 2.2: How to Get Country Code Based on Country ID

Note that although these answers might work, they're not the proper Magento Way™ to do this. The right path to choose is to use Service Contract.

The Directory module comes with a very handy Service Contract called the "CountryInformationAcquirerInterface". You can find it at Magento\Directory\Api\CountryInformationAcquirerInterface. This has a very handy function called getCountriesInfo(), which returns an array of CountryInformation data models, which - in their turn - have a method called getAvailableRegions(). So the full example would look something like this:

/**
 * @var \Magento\Directory\Api\CountryInformationAcquirerInterface
 */
protected $countryInformationAcquirer;

/**
 * Constructor call.
 * @param \Magento\Directory\Api\CountryInformationAcquirerInterface $countryInformationAcquirer
 */
public function __construct(
    \Magento\Directory\Api\CountryInformationAcquirerInterface $countryInformationAcquirer
) {
    $this->countryInformationAcquirer = $countryInformationAcquirer;
}

/**
 * Just a simple example
 * @return array
 */
public function example()
{
    $data = [];

    $countries = $this->countryInformationAcquirer->getCountriesInfo();

    foreach ($countries as $country) {
        // Get regions for this country:
        $regions = [];

        if ($availableRegions = $country->getAvailableRegions()) {
            foreach ($availableRegions as $region) {
                $regions[] = [
                    'id'   => $region->getId(),
                    'code' => $region->getCode(),
                    'name' => $region->getName()
                ];
            }
        }

        // Add to data:
        $data[] = [
            'value'   => $country->getTwoLetterAbbreviation(),
            'label'   => __($country->getFullNameLocale()),
            'regions' => $regions
        ];
    }

    return $data;
}

With most tasks in Magento 2, the best approach is to check first if there's a Service Contract that might fit your needs. Because most of the time, there is.

Here $customerAddress = $observer->getCustomerAddress(); is an object of 'Magento\Customer\Model\Address' class and this class extend from Magento\Customer\Model\Address\AbstractAddress

So go to Magento\Customer\Model\Address\AbstractAddress has some get function that help you a lot. I add a line that help you to pick country.

$this->logger->info($customerAddress->getCountry());

<?php

namespace Vendor\Module\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Event\Observer;

class CustomerAddressSaveAfterObserver implements ObserverInterface {

    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $loggerInterface //
    ) {
        $this->logger = $loggerInterface;
    }

    public function execute(Observer $observer) {
        $this->logger->debug("CustomerAddressSaveAfterObserver execute");

        $customerAddress = $observer->getCustomerAddress();
        $customer = $customerAddress->getCustomer();
        $billingAddress = $customer->getDefaultBillingAddress();
        $shippingAddress = $customer->getDefaultShippingAddress();
        // get country code
        $this->logger->info($customerAddress->getCountry());
        // get country name
        $this->logger->info($customerAddress->getCountryModel()->getName());
    }

}