The script that puts lang value in search form is the reason of validation errors as XHTML 1.0 Transitional. To fix it just add <![CDATA[" ... "//]]> around it in wp-content\plugins\polylang\include\core.php on line 453. Instead of:
And, of course, it will be good if we have an option to switch this option off if we do not use search form.
Thanks for cool plugin.