Parent page selector shows page Titles as they were last seen Live, not the current titles

[GrUnicrn]

Issue summary

This happens when translating page trees that are still (largely) in Draft.

When the page is first translated using "Translate into" button and "Copy from Canonical" checkmark, the page's title is copied from the canonical version (i.e. usually English). If the user later translates the page and changes the Title to the target language, but does not publish the translated page, its name will keep being displayed in English in "Translate into" Parent page dropdowns.

As a workaround, one may publish and then immediately un-publish the parent page.

How to reproduce?

1. Press "translate into" on a random page (let's call it A)
2. When the page editor opens, change the page title from A to B.
3. Save the draft. Do not publish the new page.
4. Translate another page into the same language. Look at the "Parent page" dropdown.

Expected result:
B is an option in the dropdown.

Actual result:
A is an option in the dropdown.

Workaround

1. Publish and immediately un-publish page B.
2. Translate another page into the same language. Look at the "Parent page" dropdown.
3. B is now an option in the dropdown, not A.

Technical details

• Django Version: 3.1.3
• Python Version: 3.8.5
• Wagtail version: 2.10
• Wagtailtrans version: d490c72