Could not set locale to esn

This seems to happen, if Spanish translation is selected.

Error 20081205064010 WARNING:   std/common.inc.php : 529 Could not set locale to 'esn'
Error 20081205064010                     index.php : 140 -> setLang("es")
Error 20081205064010 
Error 20081205064010      Variables in setLang():
Error 20081205064010                          lang = es
Error 20081205064010                        g_lang = es
Error 20081205064010                      filepath = lang/es.inc.php
Error 20081205064010                  g_lang_table = Array
Error 20081205064010                        locale = esn
Error 20081205064010                           res = NULL
Error 20081205064010    v0.08093, from 212.81.220.131,  uri:/index.php?go=projViewChanges&prj=5
Error 20081205064010 
Log 20081205064010 Damian Ruiz@212.81.220.131 -> projViewChanges /index.php?go=projViewChanges&prj=5  (Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.0.4) Gecko/2008102920 Firefox/3.0.4)