Fire hook when user clicks language button

This allows us to correctly instrument (via the
UniversalLanguageSelector instrument) this action so interaction rates
between treatments can be compared.

Bug: T273232
Bug: T273928
Change-Id: Icc64044d51c567482d4f286637366cefb7beeb3e

resources/js/ext.uls.interface.js

@@ -473,6 +473,8 @@
 				!standalone
 			);
 
+			mw.hook( 'mw.uls.compact_language_links.open' ).fire();
+
 			// Trigger the click handler to open ULS once ready
 			if ( standalone ) {
 				// Provide access to display and input settings if this entry point is the single point