Inbound routes, Other, Language...
You have to do this for each inbound route...
I am not sure but I think that adding
language = en_GB
under
Asterisk SIP settings, Chan SIP settings (if you are using it, Advanced General settings, Other SIP Settings
could also do the trick globally (as long as it is not overriden by the route, ie set to "default")...
As for the forum admin I guess @jfinstrom could most likely help there...
Good luck and have a nice day!
Nick