I was trying to figure out how to get translations working on my system. I had modified the en_US messages.po file and then ran msgfmt message.po (after installing the gettext package) to compile it to messages.mo. Problem was the changed text was no showing up. After a bunch of searching and trial and error around I found this page:
http://ca2.php.net/manual/en/book.gettext.php#91187. Basically it says that it will only support the installed languages on your system. I only have en_US.utf8 and common/lib/languageSettings.php has English as en_US.iso88591. When I changed iso88591 to utf8 it started working.
Hopefully this helps other who are trying to get this working.