Comment 8 for bug 1933109

vlgrevtsev, could you check the default environment variables that systemd sets? It should be something like this on a Juju-provisioned machine:

sudo systemctl show-environment
LANG=C.UTF-8
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin

Juju itself does not alter cloud-init defaults so cloud-init should just set the locale to C.UTF-8 and it will be used by all daemons spawned by it:

2021-06-22 16:00:28,183 - handlers.py[DEBUG]: start: modules-config/config-locale: running config-locale with frequency once-per-instance
2021-06-22 16:00:28,183 - util.py[DEBUG]: Writing to /var/lib/cloud/instances/cb2fb8ce-ff5a-43b1-ab7d-dec891b503e9/sem/config_locale - wb: [644] 25 bytes
2021-06-22 16:00:28,184 - helpers.py[DEBUG]: Running config-locale using lock (<FileLock using file '/var/lib/cloud/instances/cb2fb8ce-ff5a-43b1-ab7d-dec891b503e9/sem/config_locale'>)
2021-06-22 16:00:28,184 - util.py[DEBUG]: Reading from /etc/default/locale (quiet=False)
2021-06-22 16:00:28,184 - util.py[DEBUG]: Read 13 bytes from /etc/default/locale
2021-06-22 16:00:28,219 - cc_locale.py[DEBUG]: Setting locale to C.UTF-8
2021-06-22 16:00:28,219 - debian.py[DEBUG]: System has 'LANG=C.UTF-8' requested 'C.UTF-8', skipping regeneration.
2021-06-22 16:00:28,219 - handlers.py[DEBUG]: finish: modules-config/config-locale: SUCCESS: config-locale ran successfully

As a result, daemons started via systemd should get the correct locale settings:

sudo cat /proc/<pid>/environ

The problem may be more involved since python processes in our case are started by Apache and run as WSGI daemon processes (not managed by systemd).