Well I thought it would be tough but it’s actually pretty simple, or even a one line change if you want to switch back to English. To do the latter, simple open your wp-config.php file, find the WPLANG constant, and empty it. Period.

define('WPLANG', '');

Now if you need to switch to another language than English, you will need to grab the language files on the WordPress Language File Repository, and upload them to your install before modifying the wp-config.php file.

To do so, go to the repository, chose the desired language (for examplefr_FR), click the branches link, pick the version of your WP install, and open the messages repertory. Download all the files you see, and upload them to the wp-content/languages repertory on your server. Now you can edit your wp-config.php file and change the WPLANG value to the language code you picked (in my case fr_FR).

define('WPLANG', 'fr_FR');

That’s it. :)