User account form ignores preferred language field default values

Spin-off from #2405943-31: User entity validation misses form validation logic → and #2227381-48: Apply formatters and widgets to User base fields 'name' and 'email' → :

Problem/Motivation

    $user_preferred_langcode = $register ? $language_interface->getId() : $account->getPreferredLangcode();
    $user_preferred_admin_langcode = $register ? $language_interface->getId() : $account->getPreferredAdminLangcode(FALSE);

The form has hard-coded defaults for the language fields here, which in case of the preferred admin langcode does not match the field definition default value (did not check the other one). The form should respect the default values of the fields though. I'm not sure which default value is the right one though.

Note that #2227381: Apply formatters and widgets to User base fields 'name' and 'email' → would probably address the problem also, but I guess the right default value could need some discussion as well.

Proposed resolution

Remaining tasks

User interface changes

API changes