Changing the default language on a Drupal site can cause a variety of issues. Most importantly it changes the assumption in Drupal about all the content and configuration that does not have a language explicitly assigned. Such as views, the site name, contact form configuration, etc. Most people who want to change their default language only want to do so to change how the language negotiation works, and they have no intention to translate / swap all configuration/content that does not have language assigned to the new language.
We observed repeated issues from people who just wanted to change the language negotiation configuration by changing the default site language and broke their translation processes due to the changed assumptions about their language. This is a very common problem. There is a contributed module written to work around this problem by introducing another configurable language negotiation piece: http://drupal.org/project/fallback_language_negotation. It is not a very well known module but solves the problem nicely, allowing the site to fallback to a language in the negotiation process without actually changing the default language.
Therefore we introduced a "Selected language" option in place of the "Default language" option in language negotiation in Drupal 8, making it possible to deviate from the overall site default for the language negotiation fallback, essentially porting the fallback_language_negotiation module to core.
BEFORE (configure link on the default language row went to the page to *actually* change the site default language):
AFTER part 1: negotiation page
AFTER part 2: configuration page for selected language
Now the configure link in the new selected language row, see part 1 image above, goes here and a language can be selected without changing the actual site default language