Index: django/db/models/fields/__init__.py
===================================================================
--- django/db/models/fields/__init__.py	(revision 7455)
+++ django/db/models/fields/__init__.py	(working copy)
@@ -669,7 +669,7 @@
 
     def _format(self, value):
         if isinstance(value, basestring):
-            return value
+            return value.replace(",", ".")
         else:
             return self.format_number(value)