Mailman's character set for English is us-ascii. This causes non-ascii characters in posts and message headers and footers and other places to be replaced by question marks and/or numeric HTML entities in various contexts if the web interface and in plain format digests. This can be avoided by changing Mailman's character set for English to UTF-8. This has not been done in the GNU Mailman distribution for mainly superstitious reasopns, but it is believed to be safe. This is done by placing the line

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

in mm_cfg.py and restarting Mailman.