Arch Linux

$ LC_ALL=foo locale
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=en_XX.UTF-8@POSIX
LC_CTYPE="foo"
LC_NUMERIC="foo"
LC_TIME="foo"
LC_COLLATE="foo"
LC_MONETARY="foo"
LC_MESSAGES="foo"
LC_PAPER="foo"
LC_NAME="foo"
LC_ADDRESS="foo"
LC_TELEPHONE="foo"
LC_MEASUREMENT="foo"
LC_IDENTIFICATION="foo"
LC_ALL=foo

$ locale
LANG=en_XX.UTF-8@POSIX
LC_CTYPE=en_US.UTF-8
LC_NUMERIC="en_XX.UTF-8@POSIX"
LC_TIME="en_XX.UTF-8@POSIX"
LC_COLLATE="en_XX.UTF-8@POSIX"
LC_MONETARY="en_XX.UTF-8@POSIX"
LC_MESSAGES="en_XX.UTF-8@POSIX"
LC_PAPER="en_XX.UTF-8@POSIX"
LC_NAME="en_XX.UTF-8@POSIX"
LC_ADDRESS="en_XX.UTF-8@POSIX"
LC_TELEPHONE="en_XX.UTF-8@POSIX"
LC_MEASUREMENT="en_XX.UTF-8@POSIX"
LC_IDENTIFICATION="en_XX.UTF-8@POSIX"
LC_ALL=

2.21-4 seems to be broken too: https://bbs.archlinux.org/viewtopic.php?pid=1552712#p1552712