Go to Appearance > Menus
❶ You have to create one menu per language and save them.
❷ Under ‘Menu Settings’ assign your menus to the relevant theme location, for example ‘My English Menu’ to ‘Primary menu English’ and ‘My French Menu’ to ‘Primary menu Français’.
❸❹❺ You have the possibility to add a language switcher anywhere in a menu. If you don’t see the language switcher metabox, check that it is not disabled in the screen options.
Note that the language switcher will display a language only if at least one post or one page has been published in this language.