I just hasd the same issue and have found an easy solution. It's all in the error message, especially the line:
LANGUAGE = "en_DE:en_US:en_GB:en",
When you either "apt-get install locales" or "dpkg-reconfigure locales", you are asked to select the required locales from a large list.
I went through and selected the ones that I needed. The error still appeared. I then redid it and made sure that the languages listed were the ones I selected, which would be "en_DE", "en_US" and "en_GB" in your case. Once I did that the errors vanished.