In order to enable multilingual menu first you will need to support more than one languages on your website.
If you didn't done that please check this article. This step is a must.
Next step is to enable the added language as a supported language in the website configuration. Check this article.
When you've done with this configuration open the Settings page (Setup) and enable the option Enable multi language menus and save the changes.
-- Will be helpful if you backup - export your complete Database now. Something can go wrong here.
Now on top of the settings page you will see the option for making the all restaurant's menus in the website's default language. Click on the red button.
Now after this option is enabled successfully every restaurant will have this option in their menu page.