Since Ubuntu 14 LibreOffice and Thunderbird are using English. Since Version 15, at least Thunderbird is in German again, but Office is inconvenient. I tried changing it with Tools > Options > Settings but I can only choose English or English (USA)
It could be fixed like this.
Then select Install / Remove Languages. After the window pops up, select German(not Deutsch)
After the setting, go to Libre Office, and you will see the language German(Again, notDeutsch). (GUI Language should be translated)