Thank you for the link.
Google adds alt text from your logo, I would assume that it tries to add it because of different language or maybe some other purpose.
I would suggest to change logo alt text to something different, you can do it in templates/base/scripts/BxBaseFunctions.php file (it's better to copy whole function to your template file - templates/tmpl_XXX/scripts/BxTemplFunctions.php):
function genSiteLogo()
{
global $dir, $site;
$sFileName = getParam('sys_main_logo');
if (!$sFileName || !file_exists($dir['mediaImages'] . $sFileName))
return '<a class="mainLogoText" href="' . BX_DOL_URL_ROOT . 'index.php' . '">' . getParam('site_title') . '</a>';
return '<a href="' . BX_DOL_URL_ROOT . '"><img src="' . $site['mediaImages'] . $sFileName . '" class="mainLogo" alt="your unique text" /></a>';
}