German translation: Improvements and Updates

This PR includes:
* Streamlined german translation with fitting git-specific terms (I finally found the [official git translation](https://github.com/git/git/blob/master/po/de.po))
* Updates to the german translation as well
* One new string (`CODE_INSPECTION_PROBLEMS`)
* Changes to the english nls file :grin: (one spelling mistake, one wording change, one additional hint)
* Formatting for filenames (and the Mustache template file changes needed therefore)

Briefly, this makes every translation better, but mostly the german (finally!)