WordPress/wp-includes/wp-l10n.php

<?php
require_once(ABSPATH . 'wp-includes/streams.php');
require_once(ABSPATH . 'wp-includes/gettext.php');

function get_locale() {
	global $locale;

	if (isset($locale))
		return $locale;

	// WPLANG is defined in wp-config.
	if (defined('WPLANG')) {
    $locale = WPLANG;
	}
	
	if (empty($locale)) {
    $locale = 'en_US';
	}

	$locale = apply_filters('locale', $locale);

	return $locale;
}

// Return a translated string.    
function __($text, $domain = 'default') {
	global $l10n;

	if (isset($l10n[$domain])) {
		return $l10n[$domain]->translate($text);
	} else {
		return $text;
	}
}

// Echo a translated string.
function _e($text, $domain = 'default') {
	global $l10n;

	if (isset($l10n[$domain])) {
		echo $l10n[$domain]->translate($text);
	} else {
		echo $text;
	}
}

// Return the plural form.
function __ngettext($single, $plural, $number, $domain = 'default') {
	global $l10n;

	if (isset($l10n[$domain])) {
		return $l10n[$domain]->ngettext($single, $plural, $number);
	} else {
		return $text;
	}
}

function load_textdomain($domain, $mofile) {
	global $l10n;

	if (isset($l10n[$domain])) {
		return;
	}

	if ( is_readable($mofile)) {
    $input = new FileReader($mofile);
	}	else {
		return;
	}

	$l10n[$domain] = new gettext_reader($input);
}

function load_default_textdomain() {
	global $l10n;

	$locale = get_locale();
	$mofile = ABSPATH . "wp-includes/languages/$locale.mo";
	
	load_textdomain('default', $mofile);
}

function load_plugin_textdomain($domain) {
	$locale = get_locale();
	
	$mofile = ABSPATH . "wp-content/plugins/$domain-$locale.mo";
	load_textdomain($domain, $mofile);
}

function load_theme_textdomain($domain) {
	$locale = get_locale();
	
	$mofile = get_template_directory() . "/$locale.mo";
	load_textdomain($domain, $mofile);
}

?>