I have spend a few hours for debugging and searching the failure for not dynamically re-loading the pages with a new declared locale for a user. I have declared a FALSE locale like en_EN. This locale does not exist. it must be 'en_GB' or 'en_US'.

If there was an error/info message that a locale named 'en_EN' cannot be loaded/created it can help to find such a silly mistake.
It' s not good to try to find next the cutted 'en' from 'en_EN' as a locale and go further.

Thanks
Stephan