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/css/ext.uls.pt-vector.less

@@ -3,6 +3,12 @@
 	background: none;
 }
 
+// T297579
+/* stylelint-disable-next-line selector-max-id */
+.mw-interlanguage-selector-disabled #p-lang-btn-sticky-header {
+	display: none;
+}
+
 // Applies old styling rules to legacy Vector
 .skin-vector-legacy {
 	@import 'ext.uls.pt.less';