copy en_GB.po to en_US.po and work freely on it
take a _serious_ look to the documentation about translations in http://wiki.amule.org
and feel free to post your translations whenever you go updating them. you're wellcome as a US translator
in fact, i suspect the default language isn't english: it's some kind of foreigners-for-all-the-world-english xD
and, btw, please provide an e-mail where you can be contacted. you can private message me if you don't want it to be public.
Greetings and thanks!