Polylang 1.7.9 is a maintenance release focusing on the compatibility with WordPress 4.3 which is planned to be released tomorrow.
Indeed, WordPress 4.3 will introduce several conflicts with earlier versions of Polylang. This mainly concerns the menus and the customizer. This new version aims at fixing these conflicts.
However, the language switcher is not yet compatible with the new customizer menus. There is still some work to be done in WordPress to allow plugins to have menu items with custom forms (as done by Polylang for the language switcher). Of course, you can still add a language switcher menu item in Appearance->Menus as usual.
Under the hood, WordPress 4.3 will eliminate all shared taxonomy terms. Not all users have shared terms in the database but for those who have, this impacts Polylang. The compatibility code is there since Polylang 1.7 and the core team helped a lot to make things happen as smoothly as possible, but making a database backup before upgrading to WordPress 4.3 is well recommended.
Finally this version fixes also a conflict with Yoast SEO sitemaps (when using multiple domains) and improves the usage of hreflang in the alternate links added by Polylang in the html head.