I have also noted that and I suspect that there is a flaw in Asterisk 1.4 that don’t honor the language setting when an anonymous call is received.
You can try to edit /etc/asterisk/extensions.conf (yes, I know, do no edit this file…) so that it looks like this:
[from-sip-external]
;give external sip users congestion and hangup
; Yes. This is _really_ meant to be _. - I know asterisk whinges about it, but
; I do know what I'm doing. This is correct.
exten => _.,1,NoOp(Received incoming SIP connection from unknown peer to ${EXTEN})
exten => _.,n,Set(CHANNEL(language)=en_GB)
exten => _.,n,Set(DID=${IF($["${EXTEN:1:2}"=""]?s:${EXTEN})})
exten => _.,n,Goto(s,1)Add the line exten => _.,n,Set(CHANNEL(language)=en_GB) to explicit set the language to en_GB.
Test that and report the outcome.