Hi @dicko,
Your explanation is correct but not related to the problem of adding another language to the Asterisk sound files. The problem in the Freepbx gui is that although the default language is en and the fallback is set to en, the value of language field in the sip.conf file does not changes if you are adding only one custom language (it, he, fr etc.) to the Freepbx module.
You need to setup the en language in the custom part of the module as well to change the language value in the sip.conf file.
Thank you,
Daniel Friedman
Trixton LTD.