code: public static function registerTranslations($baseDir, $localesDir, $prefix = null)
      {
          if (self::$isInit === true) {
              $dir = $baseDir . DIRECTORY_SEPARATOR . $localesDir;
              if (is_dir($dir) === false) {
                  throw new InvalidDirectory('directory "' . $dir . '" can\'t be identified');
              }
              $language = self::$currentLanguage;
              if (($pos = strpos($language, '-')) !== false) {
                  $pos = (int) $pos; // re: https://github.com/phpstan/phpstan/issues/647
                  $language = substr($language, 0, $pos);
              }
              if (($pos = strpos($language, '_')) !== false) {
                  $pos = (int) $pos; // re: https://github.com/phpstan/phpstan/issues/647
                  $language = substr($language, 0, $pos);
              }
              if (is_readable($dir . DIRECTORY_SEPARATOR . $language . '.yml') === true) {
                  $newOnes = Config::get($dir . DIRECTORY_SEPARATOR . $language . '.yml');
                  if (is_array($newOnes) === true) {
                      if ($prefix != null) {
                          $newOnes = array($prefix => $newOnes);
                      }
                      self::$translations = array_merge(self::$translations, $newOnes);
                  }
              } else {
                  Logger::get()->notice('can\'t find language "' . $language . '" in directory "' . $dir . '"');
              }
          }
      }
docs: Load translations from directory according to current language.
  
  @static
  @throws InvalidDirectory if directory can't be identified
  @param  string           $baseDir    application directory path
  @param  string           $localesDir translations directory path
  @param  string           $prefix     prefix for all translations from this directory