Changing Language in GUI not possible

[rbcb-bedag]

Hello together,
I've built a CKAN docker image with CKAN 2.8.6 and python 2.7. I have also installed this extension with all his dependencies.
everything is working, except that the whole GUI is only in english available and i cannot change the language (there is only english in the dropdown and when manually setting/adjusting the path in the URL e.g. /de/datasets it crashes and in the logs python says: UndefinedError: 'dict object' has no attribute 'de').

Looks for me, that ckan doesn't load the languages in his "config".
i have also set the ckan.localeXXX configs and also compiled the .pot and .po files to .mo files

any suggestion what else i could try ?
Do you maybe have a Dockerfile for the OpenData Swiss CKAN Instance or snippets how to correctly build the CKAN Image with this extension ?

P.S. the creation of datasets with the 4 different Languages (de,fr,it,en) works. Only changing the GUI language doesn't work.

Thank you for your help.