I have a multilingual website with five languages. Because I wanted to extensively customize Drupal's built-in English, I also created a custom language, "U.S. English." I then disabled the built-in Drupal English; the site now uses my custom U.S. English as the default language.
However, I'm confused about which language I should select to be the "Source language" for string translations on the i18n module's admin/config/regional/i18n/strings page. Should I select my custom "U.S. English" or the built-in Drupal English? How does the default language affect string translations?
EDIT: Please note that I am asking about the i18n module's "Source language" for string translations, not the site's default language. Clearly, I need to set the default language of the site on admin/config/regional/language to my custom U.S. English; if I do not, users of the site will not see it. The "Source language" for string translations (at admin/config/regional/i18n/strings) is separate from the default language, and I don't understand whether I should set it to my custom language or the Drupal default.