in file moodle/index.php lines 113-115 or so are

print_header($SITE->fullname, $SITE->fullname, 'home', '',
'<meta name="description" content="'. s(strip_tags($SITE->summary)) .'" />',
true, '', user_login_string($SITE).$langmenu);

This creates the meta tag "description" by stripping tags from the site "summary". However, if the summary contains text for multiple languages, this causes all languages to be simply concatenated (not even a space is added). When filterall is set, the content should be run through multilang filter.