Corrected and completed German localization:

 - Made some translation in formal form (Sie) to use informal form (Du) as most others
 - Added missing translation, for new XHTML-IM filter option and others
 - Corrected a few minor spelling mistakes

Tested, works.
Rechtsbelehrung → Haftungsausschluss.
Merge pull request #343 from Natureshadow/master

Corrected/completed German translation