Return target of redirect languages in mw.uls.getFrequentLanguageList

When mw.uls.getFrequentLanguageList() is requested sometimes
the language that is a redirect returned causing other codebases (like Wikibase)
to show an invalid language to the user.

Bug: T217770
Change-Id: I49c802d584081aa5992dd0ba76144059bcac56c8

tests/qunit/ext.uls.tests.js

@@ -90,4 +90,34 @@
 			'Tagalog is one of the languages presented to users in the Philippines.'
 		);
 	} );
+
+	QUnit.test( 'Add redirect target', function ( assert ) {
+		var i, foundTagalog, languages, foundFil;
+
+		foundTagalog = false;
+		foundFil = false;
+		mw.uls.getBrowserLanguage = function () {
+			return 'fil';
+		};
+
+		languages = mw.uls.getFrequentLanguageList();
+
+		for ( i = 0; i < languages.length; i++ ) {
+			if ( languages[ i ] === 'tl' ) {
+				foundTagalog = true;
+			}
+
+			if ( languages[ i ] === 'fil' ) {
+				foundFil = true;
+			}
+		}
+		assert.ok(
+			foundTagalog,
+			'Tagalog is one of the languages presented to users when "fil" language is requested'
+		);
+		assert.notOk(
+			foundFil,
+			'"fil" language is redirected to Tagalog'
+		);
+	} );
 }() );