In case you want to use a different language for the wordpress frontend (your blog) and the backend (admin panel), here is a tip how it can be done.

Step 1: Set frontend language

Configure the language of WordPress for the language you want your frontend in. You do this in wp_config.php using the constant WPLANG:

define ('WPLANG', '');

This unsets the language, meaning the frontend will be English. This also works if you use a localized version of wordpress. If you want to set another language, do so:

define ('WPLANG', 'de_DE');

This would set the language to German. You need to have a corresponding .mo-file in wp-content/languages (in this example, wp-content/languages/de_DE.mo).

Step 2: Set the backend language

Step one will also affect the backend. If you want to have the backend in another language, there is a neat little plugin which changes the backend language for you, which I found on this forum.

<?php
/*
Plugin Name: Change backend language
Version: 0.5
Plugin URI: http://forum.wordpress-deutschland.org/konfiguration/32642-frontend-soll-englisch-sein-adminbereich-deutsch-engl-od-deut-download-nehmen.html
Description: Changes the backend language
Author: Oliver Schlöbe
Author URI: http://www.schloebe.de/
*/

function os_setAdminLang($locale) {
    if( WP_ADMIN === true ) {
        $locale = 'de_DE';
        return $locale;
    }
}

add_filter('locale', 'os_setAdminLang', 1, 1);
?>

This changes the backend language to German. In case you want another language, change ‘de_DE’ to the desired language in line 13. You need a corresponding .mo-file just like for the frontend.

Save this as something like wp-content/plugins/change_be_language.php

Then active the plugin in your backend and your backend will turn into the language you set.

Attention: Make sure there are no extra spaces in the plugin-file. Especially at the end, some editors tend to add spaces or line breaks. This will result in problems (headers cannot be sent etc.).

Hope someone finds this useful.

Update: Just found out, that the author wrote a blog post himself about this (English text at the bottom of the page)…