WPCS - WordPress Currency Switcher

WPML:

• open functions.php of your current wp theme
• drop there next PHP code:

  add_filter('wp_head', function() {
      $lang = ICL_LANGUAGE_CODE;
      global $WPCS;
      switch ($lang)
      {
          case 'bg':
              $WPCS->storage->set_val('wpcs_current_currency', 'BGN');
              break;
          case 'en':
              $WPCS->storage->set_val('wpcs_current_currency', 'EUR');
              break;
          default:
              $WPCS->storage->set_val('wpcs_current_currency', 'USD');
              break;
      }
  });

• See WPML docs about languages codes

Polylang and TranslatePress:

• open functions.php of your current wp theme
• drop there next PHP code:

  add_filter('wp_head', function() {
      $lang = get_locale();
      global $WPCS;
      switch ($lang)
      {
          case 'bg_BG':
              $WPCS->storage->set_val('wpcs_current_currency', 'BGN');
              break;
          case 'en_GB':
              $WPCS->storage->set_val('wpcs_current_currency', 'EUR');
              break;
          default:
              $WPCS->storage->set_val('wpcs_current_currency', 'USD');
              break;
      }
  });

• https://wordpress.org/support/topic/switch-currency-with-language-change-polylang/
• See Polylang docs about languages codes

GTranslate:

• open functions.php of your current wp theme
• drop there next PHP code:

  add_filter('wp_head', function() {
      $lang = isset($_SERVER['HTTP_X_GT_LANG']) ? $_SERVER['HTTP_X_GT_LANG'] : '';
      global $WPCS;
      switch ($lang)
      {
          case 'bg_BG':
              $WPCS->storage->set_val('wpcs_current_currency', 'BGN');
              break;
          case 'en_GB':
              $WPCS->storage->set_val('wpcs_current_currency', 'EUR');
              break;
          default:
              $WPCS->storage->set_val('wpcs_current_currency', 'USD');
              break;
      }
  });

• https://wordpress.org/plugins/gtranslate/