Re: language file ?
1. No, you can’t use the WordPress translation because there might be phrases that occur in bbPress that aren’t translated already for WordPress. I think you already discovered and resolved that.
2. You already figured out where to put the mo file (bb-includes/languages)
3. I have seen a couple different posts for places to host the translations, maybe this one would be OK?
I have no idea how to get it hosted “officially” at Automattic.
Thanks for translating.