Load language files on callback event

IMHO JPlugin should be doing that automatically when the plugin object is instantiated. Since it doesn't developers end up calling loadLangauge on every event handler, making performance worse overall. Oh, well, that's the subject of another PR...

plugins/authentication/facebook/facebook.php

@@ -81,6 +81,9 @@ public function onUserLoginFormFields()
 
 	public function onAjaxFacebook()
 	{
+		// Load plugin language
+		$this->loadLanguage('plg_authentication_facebook');
+
 		// Try to exchange the code with a token
 		$facebookOauth = $this->getFacebookOauth();
 		$app           = JFactory::getApplication();