When deleting a language from all versions using ContentService::deleteTranslation($contentInfo, 'locale'), if an archived version only has the language being deleted, then an error is thrown by the public API.

In the REST implementation of that method, based on users requests, code is added to the REST API to delete the version if it only has that language. This should be the public API's job.

I don't see a use-case where we'd want that exception. The method is designed to remove that translation in all versions. To delete one translation without affecting the archives, deleteTranslationFromDraft() should be used. Archived versions are anyway removed after a while.