Be sure you put the locale into /etc/locale.gen and do locale-gen after changing.
The Apache Web Server needs also be restartet!
Array
(
[0] => Array
(
[0] => textdomain
[1] => gettext
[2] => _
[3] => dgettext
[4] => dcgettext
[5] => bindtextdomain
[6] => ngettext
[7] => dngettext
[8] => dcngettext
[9] => bind_textdomain_codeset
)
[1] => $_SESSION['locale'] = en_US
[2] => gettext("Translation") = Translation
[3] => gettext("First Aid") = First Aid
)