bbPress.org

I’ve never done it myself but I believe you first have to put the translation es_ES.mo file into bb-includes/languages directory.

Then you go into bb-config.php` and change the BBLANG setting:

define('BBLANG', 'es_ES');

Here is more info:

http://www.degabriel.es/2008/04/24/traduccion-de-bbpress-09-al-castellano/

and the file:

http://www.degabriel.es/wp-content/uploads/2008/04/es_es.mo