root/tags/2.1/wp-includes/l10n.php

<?php
function get_locale() {
    global $locale;

    if (isset($locale))
        return apply_filters( 'locale', $locale );

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

    if (empty($locale))
        $locale = '';

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

    return $locale;
}

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

    if (isset($l10n[$domain]))
        return apply_filters('gettext', $l10n[$domain]->translate($text), $text);
    else
        return $text;
}

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

    if (isset($l10n[$domain]))
        echo apply_filters('gettext', $l10n[$domain]->translate($text), $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, $number);
    } else {
        if ($number != 1)
            return $plural;
        else
            return $single;
    }
}

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

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

    if ( is_readable($mofile))
        $input = new CachedFileReader($mofile);
    else
        return;

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

function load_default_textdomain() {
    global $l10n;

    $locale = get_locale();
    if ( empty($locale) )
        $locale = 'en_US';

    $mofile = ABSPATH . LANGDIR . "/$locale.mo";

    load_textdomain('default', $mofile);
}

function load_plugin_textdomain($domain, $path = false) {
    $locale = get_locale();
    if ( empty($locale) )
        $locale = 'en_US';

    if ( false === $path )
        $path = PLUGINDIR;

    $mofile = ABSPATH . "$path/$domain-$locale.mo";
    load_textdomain($domain, $mofile);
}

function load_theme_textdomain($domain) {
    $locale = get_locale();
    if ( empty($locale) )
        $locale = 'en_US';

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

?>