BuddyPress.org

– is it possible with BP to disable the “Wire” ?

Yes…this can be disabled per group at the group creators discretion.

– is it possible to use P2-theme in your “Group-blog” ?

I don’t know, I have not use the P2 theme yet, but I imagine it should be no problem.

– are there any language-related wording in your plugin (at the user-interface) which need to be translated when I would like to run your plugin for a german-language-website ?

Yes, a language file is included which you may translate using the wordpress guidelines.