Fix javascript error with language fallbacks

Fallbacks are handled with ApiULSLocalization.

Bug: 51923

Change-Id: I9f68e0744bf0c730aa3a410901a9b536af7623fc

resources/js/ext.uls.i18n.js

@@ -31,6 +31,9 @@
 		locale: mw.config.get( 'wgUserLanguage' )
 	} );
 
+	// ApiULSLocalization handles fallback in ULS
+	$.i18n.fallbacks = {};
+
 	mw.uls.loadLocalization = function ( locale ) {
 		var i18n = $.i18n();