[META] Support language switcher and multilingual

Problem/Motivation

• As an administrator, the system supports a multilingual instance.
• As an administrator, I can add / edit / remove languages from the multilingual instance. This is documented.
• As an administrator, I can translate strings and labels on the website
• As an administrator, I can translate configuration (e.g. content type fields, views, ...)
• As an administrator, I can translate taxonomy terms
• As an administrator, I can translate site settings
• As an administrator, the system supports multilingual sitemaps

• As a visitor of drupal.xx, I can switch the language of the website
• The language switcher is accessible.
• As a visitor, I can choose the language of the website with a language picker in the topmenu.
• As a visitor with a visual or physical impairment, I can easily use the language picker
• As a visitor, I can easily use the language picker with my reader
• Proposal: As a site visitor, when I am viewing a page that is unavailable in a different language, then the language picker informs me that the translation is not available (e.g. greyed out and not selectable, or something similar)

• As an editor, I can add translations for menu items in different languages
• As an editor, I can add the same content in different languages so news items, pages, forms, events and cases can easily be shown in all available languages
• As an editor, I can translate asymmetrically, meaning a translated node gets all the original content copied over, but can remove / add content in the translation at will, without interfering with the original node contents.
• As an editor, I can translate all fields of all content types, unless there is an explicit reason why the field should not be translatable.
• As an editor, I can add translations to content blocks

• As a member, I can easily add translations for the events that I create
• As a member, I can easily add translations for the texts on my profile
• As a member, I get the automatic announcement and communication through the website in the language I prefer

Proposed resolution

Layout builder asymmetric translation: Layout Builder AT

Remaining tasks

• #3565065: Configure content translation
• #3565238: Make views language aware
• #3570122: Implement language switcher

User interface changes

API changes

Data model changes

Technical notes

• Symmetric or Asymmetric translations?