fixes #713, use the default language to send email

and not only to build the email body message

admin/include/updates.class.php

@@ -208,8 +208,6 @@ class updates
       $content.= "\n\n".l10n('It only takes a few clicks.');
       $content.= "\n\n".l10n('Running on an up-to-date Piwigo is important for security.');
 
-      switch_lang_back();
-
       pwg_mail_admins(
         array(
           'subject' => l10n('Piwigo %s is available, please update', $new_versions_string),
@@ -223,6 +221,8 @@ class updates
         true // only webmasters
         );
 
+      switch_lang_back();
+
       // save notify
       conf_update_param(
         'update_notify_last_notification',