How to fix apt-listchanges: Can't set locale; make sure $LC_* and $LANG are correct! in APT

First, see what locales are currently set and available:

locale
locale -a

The first command shows your current settings. The second lists all available (generated) locales. If LANG and LC_* variables reference locales not in the list from locale -a, that's your problem.

This is the recommended fix on Debian/Ubuntu. Run:

sudo dpkg-reconfigure locales

A dialog will appear. Select at least one locale (en_US.UTF-8 is standard) by pressing SPACE to toggle it. Then press ENTER to confirm. In the next screen, select your preferred default locale. The system will generate the selected locales automatically.

If dpkg-reconfigure doesn't work, edit the locale generation file directly:

sudo nano /etc/locale.gen

Uncomment (remove the #) from the locale you want (e.g., en_US.UTF-8 or C.UTF-8). Save the file, then run:

sudo locale-gen

Then set it as the default:

sudo update-locale LANG=en_US.UTF-8

If you're seeing this error only over SSH, your SSH client may be forwarding unsupported locales. Check your SSH config:

grep -i sendenv ~/.ssh/config

If you see SendEnv LANG LC_*, either remove it or make sure your remote system supports the locales your local machine uses. Alternatively, restart SSH on the server:

sudo systemctl restart ssh

After making changes, run apt again:

sudo apt update

The locale warning should be gone. You can also verify with:

locale

Your LANG should now match one of the installed locales from locale -a.