on the demo we saw that anonymous had an entry in user_language with 'de'
Anonymous user language should never be written to this table. It should be persisted in user's cookie, so that several users can use the demo in their own language. - update the code that sets this preference and enforce that anonymous not allowed to be set
(In ) Fixes #3687, make sure anonymous user cannot have language set in DB and make sure not to use that option if it is already set.