diff -r 6641e941ef1e -r ff1a9aa48cfd app/django/utils/translation/trans_real.py
--- a/app/django/utils/translation/trans_real.py	Tue Oct 14 12:36:55 2008 +0000
+++ b/app/django/utils/translation/trans_real.py	Tue Oct 14 16:00:59 2008 +0000
@@ -8,18 +8,7 @@
 from cStringIO import StringIO
 
 from django.utils.safestring import mark_safe, SafeData
-
-try:
-    import threading
-    hasThreads = True
-except ImportError:
-    hasThreads = False
-
-if hasThreads:
-    currentThread = threading.currentThread
-else:
-    def currentThread():
-        return 'no threading'
+from django.utils.thread_support import currentThread
 
 # Translations are cached in a dictionary for every language+app tuple.
 # The active translations are stored by threadid to make them thread local.
@@ -161,6 +150,15 @@
 
         res = _translation(globalpath)
 
+        # We want to ensure that, for example,  "en-gb" and "en-us" don't share
+        # the same translation object (thus, merging en-us with a local update
+        # doesn't affect en-gb), even though they will both use the core "en"
+        # translation. So we have to subvert Python's internal gettext caching.
+        base_lang = lambda x: x.split('-', 1)[0]
+        if base_lang(lang) in [base_lang(trans) for trans in _translations]:
+            res._info = res._info.copy()
+            res._catalog = res._catalog.copy()
+
         def _merge(path):
             t = _translation(path)
             if t is not None:
@@ -246,7 +244,9 @@
     True = right-to-left layout
     """
     from django.conf import settings
-    return get_language() in settings.LANGUAGES_BIDI
+    
+    base_lang = get_language().split('-')[0]
+    return base_lang in settings.LANGUAGES_BIDI
 
 def catalog():
     """
@@ -383,13 +383,13 @@
 
         for lang, dirname in ((accept_lang, normalized),
                 (accept_lang.split('-')[0], normalized.split('_')[0])):
-            if lang not in supported:
+            if lang.lower() not in supported:
                 continue
             langfile = os.path.join(globalpath, dirname, 'LC_MESSAGES',
                     'django.mo')
             if os.path.exists(langfile):
                 _accepted[normalized] = lang
-            return lang
+                return lang
 
     return settings.LANGUAGE_CODE