Locale should be detected using the client

I suggest reading the HTTP_ACCEPT_LANGUAGE header to detect what languages the client would like.

e.g.

function facebook_comments_detect_language() {
  $langcode = 'en_GB';
  if (!empty($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
  	if (preg_match_all('@(?<=[, ]|^)([a-zA-Z-]+|\*)(?:;q=([0-9.]+))?(?:$|\s*,\s*)@', trim($_SERVER['HTTP_ACCEPT_LANGUAGE']), $matches, PREG_SET_ORDER)) {
  		$langcode = check_plain(str_replace('-', '_', $matches[0][1]));
  	}
  }
  
  return $langcode;
}

I'd also add a configurable option for the default locale.