per suggestion of thehelpfulone:

in /var/lib/mailman/Mailman/htmlformat.py line 300 there is "charset =".

we changed that from us-ascii to utf-8 and deleted the matching .pyc bytecode file too.. but this does not appear to fix it.

so..i would say "upstream bug"... Wikimedia-Mailing-lists channel agrees:

kjetilho> yeah, UTF-8 would be the sensible default in just about any distro

ok, so i found Defaults.py which sets a charset per language, like:

add_language('en', _('English (USA)'), 'us-ascii', 'ltr')

changing that to

add_language('en', _('English (USA)'), 'utf-8', 'ltr')

actually makes the listinfo overview page utf-8.

..but..the list descriptions of non-English lists will still appear broken, because they are not utf-8.. it depends on the language..

and languages and descriptions can bet set by list admins...

No plans to investigate further on the WM site, candidate for upstreaming.

Looks good now?
$ curl -Is "https://lists.wikimedia.org/mailman/listinfo" | grep Content-Type
Content-Type: text/html; charset=utf-8

Yep, this seems to be fixed. Thanks for marking this as such, Nemo. Bug 37817 remains unresolved, I believe.

This works only for English pages....

For Polish we still have "iso-8859-2"

radziecki$ curl -Is "https://lists.wikimedia.org/mailman/listinfo" | grep Content-Type
Content-Type: text/html; charset=utf-8
radziecki$ curl -Is "https://lists.wikimedia.org/mailman/listinfo/wikimediapl-l" | grep Content-Type
Content-Type: text/html; charset=iso-8859-2

... although the messages look UTF-8 now...