Chouby on "[Plugin: Polylang] Menus don't switch according to languages"

In Appearance->menus, you have to create one menu per language. Let's say 'menu_en' and 'menu_ru'.
In 'menu_en', you add English pages and the language switcher. In 'menu_ru', you add Russian pages and the language switcher.
Then, once menus are both saved, you click on 'manage locations' tab and you assign 'menu_en' to 'Primary menu English' and 'menu_ru' to 'Primary menu Русский'.