Custom language settings configuration is not saved when you have a language selector enabled for any content

Problem

From a standard profile English clean install

1. Enable language module
2. Go to /admin/config/regional/content-language
3. Check custom language settings for "Content"
4. Check "Show language selector on create and edit pages" for Article
5. Save configuration
6. Uncheck custom language settings for "Content" and don't uncheck "Show language selector on create and edit pages" inside article.
7. Save configuration

Now you can see that the last uncheck is not saved and "Content" is still checked.

What I expect is that any language customization for "Content" is disabled.

As side note when content translation module is enabled and you have content types marked as translatable when you uncheck "Content" in the custom language settings the content stops being translatable and everything gets unchecked as expected.

Proposed resolution

Fix the form submit so the settings are saved regardless of the language selector settings.

Remaining tasks