http://bbpress.de is not an official site, and they seem not to be interested in a good localization. The "bbPress Deutschland" makers created a language file a few months ago, but this translation is not complete, it contains a lot of typing mistakes, some terms which are easy to misunderstand for a german reader and some wrong texts too. It hasn't never been updated.
I love bbPress (and sometimes, I hate it), so I decided to create my own and better translation. I had worked only a few hours on it, and the result is a translation much better than the bbpress.de one.
The existance of a language file is a reason to use bbPress. People should know which translations are existing, and I think, they should know it from an official site. At the moment, there is no "official" german site, but only the bbpress.de. This is a project of "inpsyde", the makers of wordpress-deutschland.de. A few weeks ago, they tried to deliver a "german WordPress version" with an advertising plugin silently, which was called by some people - me included - as a kind of "plugin spam". (It wasn't announced and not marked as an advertisement for the german company LinkLift.) This attempt to create an "official looking german WordPress release with advertising" failed, there was a great resistance from the german WordPress users. This is a strong reason for me not to trust those people anymore. I want the translation to be as free as possible from the early days on.