TitleWidget: Account for non-BCP47 language codes in Intl.Collator

This fixes Special:MovePage in cbk-zam.wikipedia.org by passing a
well-formed language code using bcp47 rather than MW's own custom
code.

Bug: T228510
Change-Id: Id94bbf7b99794f944d176036d1c429d59bd7c8b2

resources/Resources.php

@@ -2289,6 +2289,7 @@ return [
 			'mediawiki.Title',
 			'mediawiki.api',
 			'mediawiki.String',
+			'mediawiki.language',
 		],
 		'messages' => [
 			// NamespaceInputWidget

resources/src/mediawiki.widgets/mw.widgets.TitleWidget.js

@@ -61,7 +61,10 @@
 		this.api = config.api || new mw.Api();
 		// Supports: IE10, FF28, Chrome23
 		this.compare = window.Intl && Intl.Collator ?
-			new Intl.Collator( mw.config.get( 'wgContentLanguage' ), { sensitivity: 'base' } ).compare :
+			new Intl.Collator(
+				mw.language.bcp47( mw.config.get( 'wgContentLanguage' ) ),
+				{ sensitivity: 'base' }
+			).compare :
 			null;
 
 		// Initialization