Miguel,
Go to /home/httpd/html/horde/config/lang.php
edit the lines:
/* The langauge to fall back on if we cannot determine one any other
way (user choice, preferences, HTTP_ACCEPT_LANGUAGE). */
$nls['defaults']['language'] = 'en_US';
change en_US to es_ES and save
Next I thought you might need to change language settings on the workstation but changing them in the internet browser will do.
For example in IE6 go to TOOLS > Options > General >Language Button and remove any language settings forcing the php app (webmail) to default to es_ES Spanish
Regards,
Jack
PS tried Firefox worked ok with no changes.