So this is what I did:

Changed email_users_user_mail_form.php

<?php
                    $meta = "language";
                    $users = mailusers_get_users($user_ID, $meta);
                    foreach ($users as $user) {
                ?>
                    <option value="<?php echo $user->id; ?>" <?php
                        echo (in_array($user->id, $send_users) ? ' selected="yes"' : '');?>>
                        <?php echo __('User', MAILUSERS_I18N_DOMAIN) . ' - ' . $user->display_name . ' -  '. $user->language; ?>
                    </option>

And changed email-users.php:

function mailusers_get_users( $exclude_id='', $meta_filter = '') {
    global $wpdb;

    $additional_sql_filter = "";

    if ($meta_filter=='') {
        if ($exclude_id!='') {
            $additional_sql_filter = " WHERE (id<>" . $exclude_id . ") ";
        }

        $users = $wpdb->get_results(
              "SELECT id, user_email, display_name "
            . "FROM $wpdb->users "
            . $additional_sql_filter );
    } else {
        if ($exclude_id!='') {
            $additional_sql_filter .= " AND (id<>" . $exclude_id . ") ";
        }
        $additional_sql_filter .= " AND (meta_key='$meta_filter') ";
        $additional_sql_filter .= "order by meta_value, display_name";
       
        $users = $wpdb->get_results(
              "SELECT id, user_email, display_name, meta_value as $meta_filter "
            . "FROM $wpdb->usermeta, $wpdb->users "
            . "WHERE "
            . " (user_id = id)"
            . $additional_sql_filter );
    }

    return $users;
}

Thanks for the tip.

What would be nice is to have a way to display just the languages, rather than user |Language

-- Jossi