Example: You want to make Japanese as the default language.
Need to add Japanese in Admin -> Settings -> Languages.
After that, just click on button in "Is default?" column to make Japanese as default language.
After that, translate theme to Russian in Admin -> Translations -> Theme translations.
Just need to go to Admin -> Plugins then deactivate plugin Language and Language Advanced.
Menu maybe duplicated.
You just need to go to Admin -> Appearance -> Menus to delete it.