Fix locale configuration issue on Arch Linux

On a new installed Arch Linux server, I come across the following problem:

# locale -a
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_COLLATE to default locale: No such file or directory
C
POSIX
# 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_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

Especially when using man command:

# man wait
man: can't set the locale; make sure $LC_* and $LANG are correct

After referring this post, I fix the issue. My /etc/locale.conf is like this:

# cat /etc/locale.conf
LANG=en_US.UTF-8

But en_US.UTF-8 is commented out in /etc/locale.gen:

#en_US.UTF-8 UTF-8

uncomment it, then run locale-gen command, all become normal.