Don't rebuild locale cache when $language is not set

... or somewhere else where $language hasn't been set yet.

I've noticed this on in Drupal 5 but I'm pretty sure it still exists in Drupal 7.

When a module calls the t() function when $locale hasn't been set yet (like in a define()), the locale cache gets rebuilt every page request. This can have a huge performance impact on a site with a lot of translated labels.

The function locale now does something like:

function locale($string = NULL, $langcode = NULL, $reset = FALSE) {
  global $language;
  static $locale_t;

  if ($reset) {
    // Reset in-memory cache.
    $locale_t = NULL;
  }

  if (!isset($string)) {
    // Return all cached strings if no string was specified
    return $locale_t;
  }

  $langcode = isset($langcode) ? $langcode : $language->language;

  // Store database cached translations in a static var.
  if (!isset($locale_t[$langcode])) {

I think it would be safer to check if $language is set in that latest if:

<code>
function locale($string = NULL, $langcode = NULL, $reset = FALSE) {
  global $language;
  static $locale_t;

  if ($reset) {
    // Reset in-memory cache.
    $locale_t = NULL;
  }

  if (!isset($string)) {
    // Return all cached strings if no string was specified
    return $locale_t;
  }

  $langcode = isset($langcode) ? $langcode : $language->language;

  // Store database cached translations in a static var.
  if (!isset($locale_t[$langcode]) && isset($language)) {

Okay, I admit every module should take care of calling t() only when locale is initialized, but this way we can make this a bit safer. Only the buggy t() call gets not translated and all the rest is translated well.