Babel breaks Locale's interface translation form

Problem/Motivation

If Babel is installed, and I try to translate strings on the Locale module's form ("interface translation"), then after clicking the "Save" button, I get the message "The website encountered an unexpected error. Try again later."

Core commit hash: 5b5471a
Babel: d9e330c

Steps to reproduce

1. drush si -y && drush en babel -y && drush php:eval "\Drupal\language\Entity\ConfigurableLanguage::createFromLangcode('hr')->save()"
2. drush uli "admin/config/regional/translate?string=Drupal announcements"
3. Translate one of the strings and hit save.
4. "The website encountered an unexpected error. Try again later."

If you set up the Drupal instance without installing Babel, saving translations here works properly
(Just replace first command with: drush si -y && drush en locale -y && drush php:eval "\Drupal\language\Entity\ConfigurableLanguage::createFromLangcode('hr')->save()")

User interface changes

API changes

Data model changes