bg_locale_test.php

<?php

// Put this file into the dirroot of your Moodle 2.6beta and execute it, e.g.
//
//  $ sudo -u apache php bg_locale_test.php
//
// Please report results to David or at MDLSITE-2580                                                                                                    

define('CLI_SCRIPT', 1);

require_once(__DIR__.'/config.php');

$requested = 'bg_BG.UTF-8';    // as defined in bg.zip::langconfig.php

if (class_exists('Collator', false)) {
    $collator = new Collator($requested);
    $errorcode = $collator->getErrorCode();
    $errormessage = $collator->getErrorMessage();
    $actual = $collator->getLocale(Locale::ACTUAL_LOCALE);
    $valid = $collator->getLocale(Locale::VALID_LOCALE);

    fputs(STDOUT, sprintf("----
requested:    %s
actual:       %s
valid:        %s
errorcode:    %s
errormessage: %s
----
", $requested, $actual, $valid, $errorcode, $errormessage));

} else {
    fputs(STDERR, 'Collator class not found.'.PHP_EOL);
    exit(1);
}

At my notebook, I am getting

----
requested:    bg_BG.UTF-8
actual:       bg
valid:        bg
errorcode:    -128
errormessage: U_USING_FALLBACK_WARNING
----

requested: bg_BG.UTF-8
actual: ru
valid: bg
errorcode: -128
errormessage: U_USING_FALLBACK_WARNING

requested: bg_BG.UTF-8
actual: bg
valid: bg
errorcode: -128
errormessage: U_USING_FALLBACK_WARNING

(macosx, php 5.4)

requested: bg_BG.UTF-8
actual: bg
valid: bg
errorcode: -128
errormessage: U_USING_FALLBACK_WARNING

macports php 5.5.5

requested: bg_BG.UTF-8
actual: ru
valid: bg
errorcode: -128
errormessage: U_USING_FALLBACK_WARNING

Linux version 2.6.32-5-amd64 (Debian 2.6.32-48squeeze3)