/usr/share/language-tools/update-langlist is in accountsservice 0.6.43-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 | #!/bin/sh -e
#
# update-langlist maintains the LANGUAGE priority list. It does so in
# a simplified manner, unlike the principal UI for setting the user
# language which provides full control.
first_language=$1
homedir=$2
renewed_langlist=
[ -n "$first_language" -a -n "$homedir" ] || exit 1
get_old_langlist() {
    if [ -r "$homedir/.pam_environment" ]; then
        while read line; do
            if [ "${line%=*}" = 'LANGUAGE' ]; then
                old_langlist=${line#*=}
                break
            fi
        done < $homedir/.pam_environment
    fi
}
old_langlist=
english=true
if [ ${first_language%%[_@]*} != 'en' ]; then
    get_old_langlist
    english=false
fi
if [ -n "$old_langlist" ] && ! $english; then
    if expr $old_langlist : ".*:$first_language:" > /dev/null ; then
        renewed_langlist=$( echo $old_langlist | sed -r "s/(.+:)($first_language:)/\2\1/" )
    elif [ ${old_langlist%%:*} = $first_language ]; then
        # no change
        renewed_langlist=$old_langlist
    else
        renewed_langlist=$first_language:$old_langlist
    fi
else
    # build fresh list with 'en' as the last element
    # no other languages needed if English was selected
    renewed_langlist=$first_language
    [ $first_language != 'en' ] && renewed_langlist=$renewed_langlist:en
fi
echo $renewed_langlist
 |