No translation for the language 'en_US' available, why ?

Question

Here is my config.ini

resources.locale.default = "en"
resources.translate.adapter = "gettext"
resources.translate.data = APPLICATION_PATH "/../languages/"
resources.translate.option.scan = "directory"
resources.translate.registry_key = "Zend_Translate"
resources.translate.disableNotices = true
resources.translate.default = en

And my languages are structured like in the manual:

application/
languages/
-- en/
---- LC_MESSAGES/
------ default.po
-- se/
---- LC_MESSAGES/
------ default.po

Any ideas ?