Install Wordpress languages from languages.koodimonni.fi with this composer script. Place it in scripts/Koodimonni/Installer.php or just copy installLanguages() function to your composer scripts and require it after install/update.

Installer.php

<?php

namespace Koodimonni;

use Composer\Script\Event;

class Installer {
  public static function installLanguages(Event $event) {
    $io = $event->getIO();
    $extra = $event->getComposer()->getPackage()->getExtra();
    if (!empty($extra['wordpress-languages-dir'])) {

      $root = dirname(dirname(__DIR__));
      $wp_languages_folder = "{$root}/{$extra['wordpress-languages-dir']}";
      $wp_languages_vendor = "{$root}/vendor/koodimonni-language";

      if (!file_exists($wp_languages_folder)) {
        $io->write("{$wp_languages_folder} Doesn't exist. If this is correct, please create it!");
        exit(1);
      }

      $io->write("Copying languagefiles into: {$wp_languages_folder}");
      //Use rsync to copy all but composer.json
      exec("rsync -va {$wp_languages_vendor}/*/* {$wp_languages_folder} --exclude=composer.json");
    }
  }
}
?>