Steps to reproduce:

• add language English (International) eng-INT
• go to Content>Ibexa Digital Experience Platform>Ibexa Platform
• Add translation eng-INT

Result:

• Richtext fields Short description and Description are not editable
• Chrome console shows error:

  RangeError: Incorrect locale information provided
      at new Locale (<anonymous>)
      at BaseRichText.init (ibexa-richtext-onlineeditor-js.js:131065:22)
      at ibexa-admin-ui-content-edit-parts-js.js:8601:20
      at NodeList.forEach (<anonymous>)
      at ibexa-admin-ui-content-edit-parts-js.js:8599:79
      at ./vendor/ibexa/admin-ui/src/bundle/Resources/public/js/scripts/fieldType/ezrichtext.js (ibexa-admin-ui-content-edit-parts-js.js:8605:3)
      at __webpack_require__ (runtime.js:23:42)
      at __webpack_exec__ (ibexa-admin-ui-content-edit-parts-js.js:10368:48)
      at ibexa-admin-ui-content-edit-parts-js.js:10369:2343
      at webpackJsonpCallback (runtime.js:157:39)
  

Note:
Same error happens when using other language codes that have a 3 letter region code - like BAR in foo-BAR