Hide sticky header language button with preference disabled

Adds a class to body when the ULS dialog is disabled.

Bug: T297579
Change-Id: I791c65c6a7ac5221dcf24e5db01c22c3e2e9f81a

resources/js/ext.uls.interface.js

@@ -562,6 +562,7 @@
 			initContentLanguageSelectorClickHandler();
 		} else {
 			$( '.mw-interlanguage-selector' ).removeClass( 'mw-interlanguage-selector' );
+			document.body.classList.add( 'mw-interlanguage-selector-disabled' );
 		}
 	}