how-to-fix-locale-error

Sometimes locale setting fails as follows after locale generation.
Example: English (en_US)

root@flora:~# update-locale LANG=en_US.UTF-8
demo@flora:~$ locale
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=ja_JP.UTF-8
LC_TIME=ja_JP.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=ja_JP.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=ja_JP.UTF-8
LC_NAME=ja_JP.UTF-8
LC_ADDRESS=ja_JP.UTF-8
LC_TELEPHONE=ja_JP.UTF-8
LC_MEASUREMENT=ja_JP.UTF-8
LC_IDENTIFICATION=ja_JP.UTF-8
LC_ALL=

Then, you should edit /etc/locale.conf as root like below.
Example for en_US:

LANG=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
LC_IDENTIFICATION=en_US.UTF-8
LC_MEASUREMENT=en_US.UTF-8
LC_MONETARY=en_US.UTF-8
LC_NAME=en_US.UTF-8
LC_NUMERIC=en_US.UTF-8
LC_PAPER=en_US.UTF-8
LC_TELEPHONE=en_US.UTF-8
LC_TIME=en_US.UTF-8

After logout & login user's session, system locale will be fixed correctly.

Note:
It is possible that the environment variables are redefined in other files than locale.conf. If you are using a desktop environment, such as GNOME, its language settings may be overriding the settings in locale.conf.

References:
https://wiki.archlinux.org/title/Locale
https://wiki.debian.org/Locale