How do I translate to English?
By default, if you have English configured, it is not available as a translation target. However, you are only one checkbox away from enabling that feature:



Go edit English and make it available for translation. This will make English behave as any other language and from then on, you can "translate" to English as well:



All strings will still fall back on the built-in English text (eg. if you also have Hungarian on the site, untranslated strings in Hungarian will fall back on Drupal built-in English text). However when English is used for display, all your overrides will be effective. (Cross-language fallbacks can be implemented in contributed modules now with Drupal 8's service architecture for string lookups).

Source: http://hojtsy.hu/blog/2013-aug-20/drupal-8-multilingual-tidbits-12-english-can-now-be-translated

Result: