Hello,
You can assign the language switcher to the menu that is already in the header builder.
If you want to add a wdiget, follow these steps:
1. Create a sidebar in Dashboard > Sidebars;
2. Add the language widget to this Sidebar;
3. Create HTML block and add Widgetized Sidebar element, chose your sidebar;
4. Add the HTML block to the Header Builder: https://xtemos.com/docs/woodmart/html-blocks-2/#header_builder
Best Regards