I encountered at least one other problem with a construction of a line:
#: Mailman/HTMLFormatter.py:78 msgid "<em>(%(num_concealed)d private member%(plu)s not shown)</em>" msgstr "<em>(%(num_concealed)d privé lid/leden niet getoond)</em>"
The dutch plural of member ('lid') is not created by adding an 's' (it's "leden".) In fact, it's generally not true that "s" makes the best plurals ("en" would be better, in Dutch, though that wouldn't fix it in this case.)
Barry, shall I just rewrite such cases to make them more easily translatable?