Why is it not recommended to change the default language? I have a fresh installation of Drupal 8.2.5 and want to use it for mostly Spanish content, but keeping the admin interface in English. /admin/config/regional/language says,


  It is not recommended to change the default language on a working site.


And /admin/config/regional/language/detection/selected says,


  Changing the site's default language itself might have other undesired side effects.


What will break if I change the default language? Or is a better question to ask, should I want to change the default language? I found this question, but it does not answer my question, aside from also being pre-Drupal8.

Thanks.
In Drupal 7 it was total chaos. With Drupal 8 things have improved a lot. It's now possible to switch default language but but there may still be effects:


URL alias has a language. Changing default language can bring some strange behavior, especially if some alias was created manually: https://www.drupal.org/node/2484411
Configuration entities can end up not having the right language. You will perhaps need to export the configuration and do some manual work like making sure the exported configuration has the right lang code.
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