In the Avast5.ini (c:\programdata\avast\ ) there is a section


[Languages]
Engine=1043

In my case this code is dutch.
Lookup your language code:
http://www.robvanderwoude.com/languagecodes.php

Maybe with creating the packages in the console, you can create an admin.ini file where this is set to the language you desire.
I never used an admin.ini file, so cant help you there for configuring, but there are several evangelists that have used it and can explain.