Are you sure you've uploaded and updated both .po and .mo files? Also make sure your WordPress language is also de_DE. Can you attache the translation files here? We'll test it as well.

when i use the phrases for translation than i can not edit all files.

see my demo page: www.minirc.de/community/

Menus are in Appearance > Menu > wpForo Navigation

The "ago" phrase is deprecated, you should translate "%s ago" phrase to "%s Tagen"

As per "You" I can't say yet, we'll check it...

Hi.. i'm a new member. And i have some problems with the german language.

On my ftp Server i see the german Files. But in the Admin Cp in Wp i can't select that.

Any idea?

You should change WordPress language to get wpForo in German language. WordPress language switcher located in Settings > General admin page: