[META] Improve Multilingual UI

Problem/Motivation

miro_dietiker:

[W]e need a new parent meta issue that cares about improving the multilingual UI.
That will be improved summary, published translation status fields, filtering elements on output, filtering on the translate tab and many more. We will need to create a special round of mock-ups also to design the resulting UX goals and sync with the old "translatable err field" wishes.

And to avoid the ui overload/clutter, we really need to have the tristate thing and should promote it.

Proposed resolution

Remaining tasks

• Create UI mock-ups.
• Improve summary.
• Add published translation status fields.
• Filtering elements on output.
• Filtering on the translate tab.
• Finish tristate (#2856008: Enable behavior plugins with a tristate).

User interface changes

API changes

Data model changes