Built-in interface language code is now defined by LANGUAGE_SYSTEM and is not 'en'

In Drupal 7 and before, the default system language for interface translation was depicted by the language code 'en'. This had various issues, so Drupal 8 breaks free of using a code of an actual language for this and instead uses 'system' (defined by the LANGUAGE_SYSTEM constant, use that!). This is important if you want to get the translation of a string and might need the original value in cases. The English translation might be different from the original string from now on.

Drupal 7 and before:

// Get untranslated version of this string.
t('some text', array(), array('langcode' => 'en'));

Drupal 8:

// Get untranslated version of this string.
t('some text', array(), array('langcode' => LANGUAGE_SYSTEM));

If you feel these examples are a bit contrived (you can just use the string without t() there, right?), you are right. Imagine situations where the langcode is dynamic.