My old configuration was confluence 2.5.4 ear/war with german language pack (loaded from your site, see attachment) installed.

I just upgraded to 2.6.0 using your upgrade documentation, without copying the language pack (2.6.0 has the german translation included). From now on it's impossible to change the default language to english, the displayed language still stays in german. Same with changing the user prefered language to english.On the other side, switching to french works.

I also tried the following

• I restarted the web container (tomcat)
• I deactivated the german language plugin
  without any visible results.

The only way to get back to english is removing the $CONFLUENCE_HOME/confluence/WEB-INF/lib/confluence-de_DE-plugin-1.1.jar file. When I reinstalled this file, I get the same behaviour as described above.