If you want to replace the (ugly) GIF with moderns SVG:

Si vous souhaitez remplacer les GIF par des SVG

<?php
/**
 * @package     Joomla.Site
 * @subpackage  mod_languages
 * @author      web-eau.net
 * @copyright   (C) 2010 Open Source Matters, Inc. <https://www.joomla.org>
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

use Joomla\\CMS\\HTML\\HTMLHelper;
use Joomla\\CMS\\Language\\Text;
use Joomla\\CMS\\Uri\\Uri;
use Joomla\\CMS\\Factory;
use Joomla\\CMS\\Language\\Associations;
use Joomla\\CMS\\Router\\Route;
use Joomla\\CMS\\Language\\LanguageHelper;
use Joomla\\CMS\\Language\\Multilanguage;
use Joomla\\Component\\Menus\\Administrator\\Helper\\MenusHelper;

$languages	= LanguageHelper::getLanguages();

function getlnglist($languages){
$user		= Factory::getUser();
		$lang		= Factory::getLanguage();
		$languages	= LanguageHelper::getLanguages();
		$app		= Factory::getApplication();
		$menu		= $app->getMenu();
		$active		= $menu->getActive();
  
  	// Get menu home items
		$homes      = [];
		$homes['*'] = $menu->getDefault('*');

		foreach ($languages as $item)
		{
			$default = $menu->getDefault($item->lang_code);

			if ($default && $default->language === $item->lang_code)
			{
				$homes[$item->lang_code] = $default;
			}
		}

		// Load associations
		$assoc = Associations::isEnabled();

		if ($assoc)
		{
			if ($active)
			{
				$associations = MenusHelper::getAssociations($active->id);
			}

			$option = $app->input->get('option');
			$component = $app->bootComponent($option);

			if ($component instanceof AssociationServiceInterface)
			{
				$cassociations = $component->getAssociationsExtension()->getAssociationsForItem();
			}
			else
			{
				// Load component associations
				$class = str_replace('com_', '', $option) . 'HelperAssociation';
				\\JLoader::register($class, JPATH_SITE . '/components/' . $option . '/helpers/association.php');

				if (class_exists($class) && \\is_callable(array($class, 'getAssociations')))
				{
					$cassociations = \\call_user_func(array($class, 'getAssociations'));
				}
			}
		}
  
$levels  	= $user->getAuthorisedViewLevels();
$sitelangs  = LanguageHelper::getInstalledLanguages(0);
$multilang  = Multilanguage::isEnabled();
// Filter allowed languages
foreach ($languages as $i => &$language)
		{
		    $language->active = ($language->lang_code === $lang->getTag());

				// Fetch language rtl
				// If loaded language get from current JLanguage metadata
				if ($language->active)
				{
					$language->rtl = $lang->isRtl();
				}
				// If not loaded language fetch metadata directly for performance
				else
				{
					$languageMetadata = LanguageHelper::getMetadata($language->lang_code);
					$language->rtl    = $languageMetadata['rtl'];
				}

				/*if ($multilang)
				{*/
					if (isset($cassociations[$language->lang_code]))
					{
						$language->link = Route::_($cassociations[$language->lang_code]);
					}
					elseif (isset($associations[$language->lang_code]) && $menu->getItem($associations[$language->lang_code]))
					{
						$itemid = $associations[$language->lang_code];
						$language->link = Route::_('index.php?lang=' . $language->sef . '&Itemid=' . $itemid);
					}
					elseif ($active && $active->language === '*')
					{
						$language->link = Route::_('index.php?lang=' . $language->sef . '&Itemid=' . $active->id);
					}
					else
					{
						if ($language->active)
						{
							$language->link = Uri::getInstance()->toString(array('path', 'query'));
						}
						else
						{
							$itemid = isset($homes[$language->lang_code]) ? $homes[$language->lang_code]->id : $homes['*']->id;
							$language->link = Route::_('index.php?lang=' . $language->sef . '&Itemid=' . $itemid);
						}
					}
			
		}
	return $languages;
}

$list = getlnglist($languages);

$wa = $app->getDocument()->getWebAssetManager();
$wa->registerAndUseStyle('mod_languages', 'mod_languages/template.css');
?>
<div class="mod-language ms-5">
	<p class="visually-hidden" id="language_picker_des_<?php echo $module->id; ?>"><?php echo Text::_('MOD_LANGUAGES_DESC'); ?></p>

<?php if ($headerText) : ?>
	<div class="mod-languages__pretext pretext"><p><?php echo $headerText; ?></p></div>
<?php endif;  ?>

<?php if ($params->get('dropdown', 0)) : ?>
	<?php HTMLHelper::_('bootstrap.dropdown', '.dropdown-toggle'); ?>
	<div class="mod-languages__select btn-grou">
		<?php foreach ($list as $language) : ?>
			<?php if ($language->active) : ?>
				<a id="language_btn_<?php echo $module->id; ?>" type="button" data-bs-toggle="dropdown" class="bt dropdown-toggle" aria-haspopup="listbox" aria-labelledby="language_picker_des_<?php echo $module->id; ?> language_btn_<?php echo $module->id; ?>" aria-expanded="false">
					<?php if ($params->get('dropdownimage', 1) && ($language->image)) : ?>
						<?php echo HTMLHelper::_('image', 'mod_languages/' . $language->image . '.gif', $params->get('full_name') ? '' : $language->title_native, null, true); ?>
					<?php endif; ?>
					<?php #echo $params->get('full_name', 1) ? $language->title_native : strtoupper($language->sef); ?>
				</a>
			<?php endif; ?>
		<?php endforeach; ?>
		<ul aria-labelledby="language_picker_des_<?php echo $module->id; ?>" class="lang-block dropdown-menu">

		<?php foreach ($list as $language) : ?>
			<?php
				$lbl = '';
				if ($params->get('full_name') === 0)
				{
					$lbl = 'aria-label="' . $language->title_native . '"';
				}
			?>
			<?php if (!$language->active) : ?>
				<li>
					<a <?php echo $lbl; ?> href="<?php echo htmlspecialchars_decode(htmlspecialchars($language->link, ENT_QUOTES, 'UTF-8'), ENT_NOQUOTES); ?>">
						<?php if ($params->get('dropdownimage', 1) && ($language->image)) : ?>
							<?php echo HTMLHelper::_('image', 'mod_languages/' . $language->image . '.gif', $params->get('full_name') ? '' : $language->title_native, null, true); ?>
						<?php endif; ?>
						<?php #echo $params->get('full_name', 1) ? $language->title_native : strtoupper($language->sef); ?>
					</a>
				</li>
			<?php elseif ($params->get('show_active', 1)) : ?>
				<?php $base = Uri::getInstance(); ?>
				<li class="lang-active">
					<a aria-current="true" <?php echo $lbl; ?> href="<?php echo htmlspecialchars_decode(htmlspecialchars($base, ENT_QUOTES, 'UTF-8'), ENT_NOQUOTES); ?>">
						<?php if ($params->get('dropdownimage', 1) && ($language->image)) : ?>
							<?php echo HTMLHelper::_('image', 'mod_languages/' . $language->image . '.gif', $params->get('full_name') ? '' : $language->title_native, null, true); ?>
						<?php endif; ?>
						<?php echo $params->get('full_name', 1) ? $language->title_native : strtoupper($language->sef); ?>
					</a>
				</li>
			<?php endif; ?>
		<?php endforeach; ?>
		</ul>
	</div>
<?php else : ?>
	<ul aria-labelledby="language_picker_des_<?php echo $module->id; ?>" class="mod-languages__list <?php echo $params->get('inline', 1) ? 'lang-inline' : 'lang-block'; ?>">

	<?php foreach ($list as $language) : ?>
		<?php
			$lbl = '';
			if ((($params->get('full_name') === 0) && ($params->get('image') === 0)) || (!$language->image))
			{
				$lbl = 'aria-label="' . $language->title_native . '"';
			}
		?>
		<?php if (!$language->active) : ?>
			<li>
				<a <?php echo $lbl; ?> href="<?php echo htmlspecialchars_decode(htmlspecialchars($language->link, ENT_QUOTES, 'UTF-8'), ENT_NOQUOTES); ?>">
					<?php if ($params->get('image', 1)) : ?>
						<?php if ($language->image) : ?>
							<?php echo HTMLHelper::_('image', 'mod_languages/' . $language->image . '.gif', $language->title_native, array('title' => $language->title_native), true); ?>
						<?php else : ?>
							<span class="label" title="<?php echo $language->title_native; ?>"><?php echo strtoupper($language->sef); ?></span>
						<?php endif; ?>
					<?php else : ?>
						<?php echo $params->get('full_name', 1) ? $language->title_native : strtoupper($language->sef); ?>
					<?php endif; ?>
				</a>
			</li>
		<?php elseif ($params->get('show_active', 1)) : ?>
			<?php $base = Uri::getInstance(); ?>
			<li class="lang-active">
				<a aria-current="true" <?php echo $lbl; ?> href="<?php echo htmlspecialchars_decode(htmlspecialchars($base, ENT_QUOTES, 'UTF-8'), ENT_NOQUOTES); ?>">
					<?php if ($params->get('image', 1)) : ?>
						<?php if ($language->image) : ?>
							<?php echo HTMLHelper::_('image', 'mod_languages/' . $language->image . '.svg', $language->title_native, array('title' => $language->title_native), true); ?>
						<?php else : ?>
							<span class="badge bg-primary" title="<?php echo $language->title_native; ?>"><?php echo strtoupper($language->sef); ?></span>
						<?php endif; ?>
					<?php else : ?>
						<?php echo $params->get('full_name', 1) ? $language->title_native : strtoupper($language->sef); ?>
					<?php endif; ?>
				</a>
			</li>
		<?php endif; ?>
	<?php endforeach; ?>
	</ul>
<?php endif; ?>

<?php if ($footerText) : ?>
	<div class="mod-languages__posttext posttext"><p><?php echo $footerText; ?></p></div>
<?php endif; ?>
</div>
<style>.mod-languages__list img{width:20px;}</style>