Change WooCommerce email locale

woocommerce-email-custom-locale.php

<?php
/**
 * Plugin Name: Slovak WooCommerce emails
 * Plugin URI: https://www.nevilleweb.sk/
 * Description: Send only slovak WooCommerce emails
 * Version: 1.0.0
 * Author: Michal Zuber
 * Author URI: https://nevilleweb.sk/
 */
if (!defined('ABSPATH')) {
    exit; // Exit if accessed directly.
}

add_filter('plugin_locale', 'custom_locale', 10, 2);
function custom_locale($locale, $domain)
{
    if ('woocommerce' == $domain && isset($_GET['wc-ajax']) && $_GET['wc-ajax'] == 'checkout') {
        return 'sk_SK';
    }

    return $locale;
}