Users in non-Latin-1 languages (language other than "en", "de", "fr", ...) will notice this bug.

Background: Starting with Perl 5.22.0, POSIX::strftime() returns Unicode (utf8 flag set) string under UTF-8 locale.
- https://metacpan.org/pod/release/SHAY/perl-5.26.1/pod/perl5220delta.pod#Better-heuristics-on-older-platforms-for-determining-locale-UTF-8ness
- https://perl5.git.perl.org/perl.git/commit/9717af6d049902fc887c412facb2d15e785ef1a4

Sympa::Language::gettext_strftime() uses it to format localized date/time, and utf8-flagged texts were mixed in the output.

Fixed by dropping utf8 flag from the result of POSIX::strftime().