Fixed #21417 -- Expanded TEMPLATE_STRING_IF_INVALID in blocktrans

Thanks keturn for the reporti, Chris Medrela for details and
Tim Graham for the review.
Refs #19915.

django/templatetags/i18n.py

@@ -150,8 +150,14 @@ class BlockTranslateNode(Node):
             else:
                 result = translation.ugettext(singular)
         default_value = settings.TEMPLATE_STRING_IF_INVALID
-        render_value = lambda v: render_value_in_context(
-            context.get(v, default_value), context)
+
+        def render_value(key):
+            if key in context:
+                val = context[key]
+            else:
+                val = default_value % key if '%s' in default_value else default_value
+            return render_value_in_context(val, context)
+
         data = dict((v, render_value(v)) for v in vars)
         context.pop()
         try: