How to revert the demo sites to English? (Or change to another language)
You can change the language in 

Administer > Localization > Languages, Currency, Locations

and change the default language (first field) to English (United States)

However, as the site is in a different language, it might be hard to find (the url is civicrm/admin/setting/localization), so here are the direct links:


Drupal
Joomla
Wordpress