Why is it not recommended to change the default language?
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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