translate interface / t does not pick up strings

I cannot reproduce the error inside a site, these are the steps I followed:

a) Enable 'locale' and 'content translation' modules.
b) Import 'French language' and enable it.
c) Create a custom language: name=>prova, prefix=>pp, code=>pp.
d) Create translation of string 'Log out' to invented lang.
e) Create some content and call: t('Log out', Array(), 'pp'). It works ok.

About t() returning the string not found I think this is because of locale function:

 return ($locale_t[$langcode][$string] === TRUE ? $string : $locale_t[$langcode][$string]);

edit: Confirmed, t() will not translate $language->name to $language->native name.

This is a simple performance feature. If the site language is English, *nothing* is passed through from t() to locale() for translation. This is how it is always been: http://api.drupal.org/api/function/t/6

...
elseif (function_exists('locale') && $langcode != 'en') {
    $string = locale($string, $langcode);
}
...

In Drupal, everything on the built-in interface is assumed to be in English by default, so if you see the site in English, there is no need to bother the database with translating your string.

Well, I have this testing code. I run it on a site with English (as default) and German also set up:

require_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

// Drop all pre-existing data.
db_query('DELETE FROM {locales_target}');
db_query('DELETE FROM {locales_source}');

// Show what languages we have.
$default = language_default();
$result = db_query('SELECT name, language FROM {languages}');
while ($row = db_fetch_object($result)) {
  echo $row->name . ($row->language == $default->language ? '-Default' : '') .'<br />';
}

// Should not enter this into the DB, if we use English.
t('test-foo');
// Should enter this to the DB.
t('test-bar', array(), 'de');
// Enters into the DB (although non-existent language).
t('test-baz', array(), '-');

// Show what strings we have.
$result = db_query('SELECT source FROM {locales_source}');
while ($row = db_fetch_object($result)) {
  echo $row->source .'<br />';
}

All runs fine, test-foo does not end up there, test-bar and test-baz does. I cannot spot an error here.

I have added some additional cleanup, and committed this one. The background info is that we discussed this test (and my testing code above) with chx on Skype, and come down to the realization that Drupal works fine, but simpletest does not reload the Drupal instance after the form submission, so the old locale data is used in locale(). This reset parameter allows simpletest to behave, pass with the locale test and aligns locale() with other in-memory cache employing functions.

Committed.