Index: main.py
===================================================================
--- main.py	(revision 7937)
+++ main.py	(working copy)
@@ -288,6 +288,9 @@
             preferred = req.session.get('language', req.languages)
             if not isinstance(preferred, list):
                 preferred = [preferred]
+            default_lang = self.env.config.get('trac', 'lang')
+            if len(default_lang):
+                preferred = [default_lang,] + preferred
             return Locale.negotiate(preferred, available, sep='-')
 
     def _get_timezone(self, req):