Multiple bugs when translating content with user preferred language settings

Problem/Motivation

I have encountered multiple issues/bugs combining Content translations and User preferences. The following steps were tested on a clean Drupal 8.6.2 installation with English as default language and default admin user.

Steps to reproduce:

• Create a basic page
• Enable Configuration Translation, Content Translation, Interface Translation and Language
• Add Swedish language at /admin/config/regional/language/add
• At /admin/config/regional/content-language select Content, Custom block and Custom menu link. Select all inner categories for each of them and save.
• At /admin/config/regional/languages/detection check the box for Account administration pages and save
• Go to /admin/people and edit the admin user. In the Language preferences, select English as the Administration pages language and save
• Go to /admin/content click edit on the previously created basic page, then select the Translate tab and add a Swedish translation. Change the title and body, click save(this translation). After saving the translation, the original english page is displayed
  and a green notice says "basic page xx has been updated". BUG: Add /sv at the end of the URL before /node/1 and the whole site changes to Swedish except for the title and content which remain in english (expected to show the swedish translation)
• BUG: After the previous steps, go to admin/content and you'll see both translations, click edit on the Swedish version when the editor opens, the English version opens instead of the Swedish version, despite /sv being present in the URL (Expected to edit the Swedish translation).
• BUG: Again at /admin/content open the dropdown operations menu for the Swedish translation and select delete, then Drupal asks you to confirm deleting both English and Swedish translations (expected to delete only Swedish translation)
• Other strange behaviour:
  • From /admin/content follow the link to the Swedish translation (You should see the entire site in Swedish except the title and body, which remain in English). Copy the URL, open up a private window in your browser and navigate to the copied URL, you should see the site correctly displayed in Swedish as well as the title and content.
  • When editing the Swedish translation, if you uncheck the "Published" box and save, you will then see that the English version as been unpublished instead (expected to unpublish the Swedish translation)

Proposed resolution

None for now

Remaining tasks

None for now