Polylang Pro 3.6.4

Pro: Adapt duplication and synchronization of the gallery block refactored in WP 5.9
Fix UI glitch in the classic editor custom fields form when changing a post language in WP 5.9 #970

Fix user description escaping #934
Fix dismissable notice when creating a term in WP 5.9 #936
Fix empty search not handled correctly. Props Dominik Schilling #937
Fix warning occurring when a 3rd party plugin attempts to register anything else than a string using the WPML API #942
Fix Yoast SEO columns not corectly drawn when quick editing a post #943

* Pro: Fix parent page not filtered by language in the block editor since WP 5.6
* Pro: Fix XLIFF mime type for PHP 7.0 and PHP 7.1
* Fix settings page displaying the media modules whne no language are defined
* Enforce Yoast SEO to use dynamic permalinks #882
* Yoast SEO: Fix static front page and blog page breadcrumb