Bug#737547: The patch you asked for is enclosed.

[Juan Rafael Fernández García]

Warning: I'm no developer, just copied and pasted from work by a
different person.

Hope it's OK. Regards

diff --git a/babel/messages/catalog.py b/babel/messages/catalog.py
index 501763b..e26e8f0 100644
--- a/babel/messages/catalog.py
+++ b/babel/messages/catalog.py
@@ -349,7 +349,10 @@ class Catalog(object):
         else:
             headers.append(('Language-Team', self.language_team))
         if self.locale is not None:
+            headers.append(('Language', str(self.locale)))
             headers.append(('Plural-Forms', self.plural_forms))
+        else:
+            headers.append(('Language', 'LANGUAGE'))
         headers.append(('MIME-Version', '1.0'))
         headers.append(('Content-Type',
                         'text/plain; charset=%s' % self.charset))