Re: language file ?
These instructions are for WordPress but the procedure is the same:
https://codex.wordpress.org/Translating_WordPress
I think you take that pot file I linked to and use a tool like poedit to translate all the phrases that are in that pot file.
Then, according to the config.php:
// Change this to localize bbPress. A corresponding MO file for the
// chosen language must be installed to bb-includes/languages.
// For example, install de.mo to bb-includes/languages and set BBLANG to 'de'
// to enable German language support.
define('BBLANG', '');
So, according to that, use poedit to generate the mo file (maybe it’s ar.mo for arabic?) and drop the ar.mo file you just created into the bb-includes/languages folder. Now change the BBLANG line to read:
define('BBLANG', 'ar');
I THINK.
I am guessing on the ar as the proper language according to this page I found:
http://www.iana.org/assignments/language-subtag-registry
I am totally guessing at all of this, but I think that’s pretty close to what you need to do. When you’re done translating, please post back and share your work. Thanks.