publisher: export site_lang substitution variable

wcs/qommon/publisher.py

@@ -809,6 +809,7 @@ class QommonPublisher(Publisher):
             'site_theme': get_cfg('branding', {}).get('theme', self.default_theme),
             'site_url': self.get_frontoffice_url(),
             'site_url_backoffice': self.get_backoffice_url(),
+            'site_lang': get_request().language or 'en',
             'today': strftime.strftime(misc.date_format(), time.localtime()),
             'now': misc.localstrftime(time.localtime()),
         }