Our Project offers a German translation for all Trinity DBs! We provide you lots of update files. For Mangos DBs please see https://sourceforge.net/projects/gm-db/
Don't know if this project is alive. However there are faults in the SQL locales file. For table locales_quest you need to change 'entry' to 'Id'. For table locales_npc_text you need to change 'entry' to 'ID'. Unfortunately I can't open a ticket here.
Stable and works.