Hello Ali, it confuses me a bit when you say all sound files. all you need in that directory is the french files from a2billing then, you need digits, letters and phonectics in different directories like /var/lib/asterisk/sound/digists/fr and put your french digits there and so on. you can find them if you need them here.
http://www.voip-info.org/wiki/view/Aste ... nal#French.
For your main question, there is a force language statement in version 1.3.0 a2billing.conf is line 483 just force your FR in there and test it.
I use bilingual so don't know if another change is needed in the conf file but I'm pretty sure that will do it. Maybe someone else can come to our aid here if you need any other changes. I would leave the above line 478 set to NO and that should do it. Reboot after the changes.
Hope this helps you.