I used to think that this was really difficult, but then I had to do some translation work for freePBX, and following these instructions, it turned out to be relatively easy.
The instructions are included in the a2billing tar file, but here is a quick over view.
1. download poedit for windows
2. Get the .po file and edit it in poedit adding in the translations.
3. Create the .mo file in poedit simply by saving.
4. create a directory in the appropriate i18n directory, named with the code of the language you want to translate, the language codes for your system on Centos are to be found in /usr/share/locale/locale.alias
5. Create an LC_Messages directory under the language code.
6. Put your new .po and .mo files in this directory.
7. Give the language directory the same user, group and permissions as all the other language files
8. Restart apache.
There is also a line of code that you are going to have to add somewhere to make the language available in the Gui, maybe someone who knows the code will chime in here.
When you have a good translation, send it to Areski, and we can get it included.