Polylang Pro 3.6.5

* Pro: Fix translation of CPTUI plural label and description not working
* Add Spanish (Ecuador) to the list of predefined languages
* Fix typo in "WordPress" string translation group. Props Viktor Szépe #682

* Fix PHP notice: Undefined property: PLL_Cache_Compat:options with cache plugins. Props bahaa-almahamid. #658
* Fix title of the search results page with Yoast SEO > 14.0

* Add compatibility with WordPress 5.6
* Pro: Add locale fallback used when the theme or plugins translations are not available
* Pro: Fix SSO and browser preferred language redirect when using multiple domains
* Pro: Fix post slugs for German and Danish in the REST API
* Pro: Fix a fatal error in ACF integration when saving url modifications with multiple domains
* Pro: Fix a deprecated notice fired by ACF since the version 5.9.2
* Pro: Fix ACF relationship fields not reloaded when changing the language in the classic editor
* Update plugin updater to version 1.8
* Add Lower Sorbian to the list of predefined language
* Options are now translated on backend when using the admin language filter
* Keep previous translations when modifying an option value
* Add navigation markup to the language switcher widget
* Fix canonical redirect for taxonomy terms
* Fix a fatal error when deleting a post with a translation group corrupted in the database
* Fix a fatal error when switching to plain permalinks and using multiple domains
* Fix a conflict with WP Sweep which could corrupt languages
* Fix title displayed instead of meta description with Yoast SEO > 14.0
* Fix PHP Notice: Undefined index: wp_the_query in /frontend/choose-lang-content.php on line 92