WPML Set Style
To set language switcher on your site header, install the latest version of WPML or Polylang plugin and adjust the basic settings.
Go to Theme Options > Header Builder and in the section, you want to add language switcher, click on add element and select this element.
Click on edit and in general settings, select your plugin.
From style tab, you can customize each item.
Now, go to WPML > Languages and from the section, Widget Language Switcher clicks on Add a new languages switcher to a widget area.
In the opened window, you can set the new main style that you want for a switcher.
Now, considering the main style of a switcher, you can adjust your main changes in style tab in header builder.