Obtained-From: http://trac.edgewall.org/changeset?format=diff&new=10052&old=10051&new_path=%2Fbranches%2F0.12-stable&old_path=%2Fbranches%2F0.12-stable

Index: trac/util/translation.py
===================================================================
--- trac/util/translation.py	(revision 9705)
+++ trac/util/translation.py	(revision 10052)
@@ -102,6 +102,8 @@
         return self.ungettext(singular, plural, num)
 
+has_babel = False
 
 try:
+    from babel import Locale
     from babel.support import LazyProxy, Translations
 
@@ -330,4 +332,12 @@
                 if '.' not in dirname]
 
+    def get_negotiated_locale(preferred_locales):
+        def normalize(locale_ids):
+            return [id.replace('_', '-') for id in locale_ids if id]
+        return Locale.negotiate(normalize(preferred_locales),
+                                normalize(get_available_locales()), sep='-')
+        
+    has_babel = True
+
 except ImportError: # fall back on 0.11 behavior, i18n functions are no-ops
     gettext = _ = gettext_noop
@@ -359,2 +369,5 @@
     def get_available_locales():
         return []
+
+    def get_negotiated_locale(preferred=None, default=None):
+        return None
Index: trac/web/main.py
===================================================================
--- trac/web/main.py	(revision 9708)
+++ trac/web/main.py	(revision 10052)
@@ -27,8 +27,4 @@
 import sys
 
-try:
-    from babel import Locale
-except ImportError:
-    Locale = None
 from genshi.core import Markup
 from genshi.builder import Fragment, tag
@@ -49,5 +45,6 @@
 from trac.util.datefmt import format_datetime, http_date, localtz, timezone
 from trac.util.text import exception_to_unicode, shorten_line, to_unicode
-from trac.util.translation import safefmt, tag_, _
+from trac.util.translation import _, get_negotiated_locale, has_babel, \
+                                  safefmt, tag_
 from trac.web.api import *
 from trac.web.chrome import Chrome
@@ -150,4 +147,9 @@
     default_timezone = Option('trac', 'default_timezone', '',
         """The default timezone to use""")
+
+    default_language = Option('trac', 'default_language', '',
+        """The preferred language to use if no user preference has been set.
+        (''since 0.12.1'')
+        """)
 
     # Public API
@@ -303,14 +305,10 @@
 
     def _get_locale(self, req):
-        if Locale:
-            available = [locale_id.replace('_', '-') for locale_id in
-                         translation.get_available_locales()]
-
-            preferred = req.session.get('language', req.languages)
-            if not isinstance(preferred, list):
-                preferred = [preferred]
-            negotiated = Locale.negotiate(preferred, available, sep='-')
-            self.log.debug("Negotiated locale: %s -> %s",
-                           preferred, negotiated)
+        if has_babel:
+            preferred = req.session.get('language')
+            default = self.env.config.get('trac', 'default_language', '')
+            negotiated = get_negotiated_locale([preferred, default] +
+                                               req.languages)
+            self.log.debug("Negotiated locale: %s -> %s", preferred, negotiated)
             return negotiated