7212: convert invalid dates to text on import

Make log message from [dfc3cc] use named arguments for l10n.
Thanks to Paul Franklin for finding this during code review.

gramps/plugins/importer/importxml.py

@@ -2492,8 +2492,10 @@ class GrampsParser(UpdateCallback):
             attrs = " ".join(
                 ['{}="{}"'.format(k,escape(v, entities={'"' : """})) 
                     for k,v in xml_attrs.iteritems()]))
-        LOG.warning(_("Invalid date {} in XML {}, preserving XML as text"
-            ).format(date_error.date.to_struct(), xml))
+        # TRANSLATORS: leave the {date} and {xml} untranslated in the format string,
+        # but you may re-order them if needed.
+        LOG.warning(_("Invalid date {date} in XML {xml}, preserving XML as text"
+            ).format(date=date_error.date.to_struct(), xml=xml))
         date_value.set(modifier=Date.MOD_TEXTONLY, text=xml)
 
     def start_datestr(self, attrs):