Polylang Pro 3.6.4

* Pro: Security: Fix unsafe custom style injection in navigation language switcher block

* Pro: Register a default (empty) value for the "lang" param when listing posts and terms in REST API
* Pro: Fix categories list refresh when the language of a post is changed in the block editor
* Pro: Fix store "pll/metabox" is already registered
* Add Kirghiz to the predefined list of languages #1308
* Fix incorrect flag url when WordPress is installed in a subfolder #1296
* Fix wrong home page url in multisite #1300

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