/usr/share/language-tools/language-validate 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 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 | #!/bin/sh -e
lang=$1
validated_language=
test -n "$lang" || exit 1
langtoolsdir=/usr/share/language-tools
#
# discard possible fallback languages
#
lang=${lang%%:*}
#
# remove possible encoding part
#
if [ $lang != ${lang%.utf8*} ]; then
    lang=${lang%.*}${lang#*.utf8}
elif [ $lang != ${lang%.UTF-8*} ]; then
    lang=${lang%.*}${lang#*.UTF-8}
fi
#
# make sure that the output is a valid language option
#
options=$( $langtoolsdir/language-options )
# exact match
for opt in $options; do
    if [ $opt = $lang ]; then
        validated_language=$lang
        break
    fi
done
if [ -z "$validated_language" ]; then
    langcode=${lang%%[_@]*}
    # try the "main" country code if any
    main_country=
    while read line; do
        if [ "${line%%[[:space:]]*}" = $langcode ]; then
            main_country=${line##*[[:space:]]}
            if [ $lang != ${lang#*@} ]; then
                main_country=$main_country@${lang#*@}
                is_variant=true
            fi
            break
        fi
    done < $langtoolsdir/main-countries
    if [ -n "$main_country" ]; then
        for opt in $options; do
            if [ $main_country = $opt ]; then
                validated_language=$main_country
                break
            fi
        done
    fi
    # try out fitting language option without paying regard to
    # country code or variant
    if [ -z "$validated_language" ]; then
        for opt in $options; do
            if [ "${opt%%[_@]*}" = $langcode -a $langcode != 'zh' ]; then
                validated_language=$opt
                break
            fi
        done
    fi
fi
if [ -z "$validated_language" ]; then
    validated_language='en'
fi
echo $validated_language
 |