Problem solved: It looks like it was a bug in Joomla, the error has been reported.
There is two solutions I encountered, I only tried out the first one, so I don't know whether the second one works.
- Go to the Content Language tab in the Language Manager
- Open the language en-GB for Edit
- Change the Access from Guest to Public
Save the changes
Invoke phpMyAdmin
- On the left panel double click on the database that is being used for your site
- On the left panel click on the table named xyz_languages (where xyz_ is the table prefix for your site) and on the right side you would notice rows from the table
- On the right panel, click on the link Edit that is against the row with lang_id 1
- In the resultant window on the right panel for the column Access change the value from 5 to 1
- Save the row
Note: Make a back-up of your database content before making this change.
Apparently, this should be resolved in the next release of Joomla!. Suggest reviewing the tracker http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33034 and the Github pull request https://github.com/joomla/joomla-cms/pull/2714.
Removing the (DE) and (UK) parts of the language switcher module was easy, just change the title native in the content tab of the language manager. Just make sure that the access is set to public.