404 when the browser's language is Live, but the main page for this language is in Draft / missing

[GrUnicrn]

Issue summary

When the language (say, Spanish) is set to Live, but the main page for Spanish is in Draft or non-existing, the users with Spanish language set in their browsers will encounter a 404 error when accessing the website main page.

How to reproduce?

1. Create a new language, say, Spanish. Set the Live checkbox to True.
2. Change the language of your browser to Spanish.
3. Delete the local website's django_language cookie.
4. Navigate to your site without specifying the page (so http://127.0.0.1:8000/, not http://127.0.0.1:8000/en)
5. Translate the English main page to Spanish, but keep the Spanish page in Draft
6. Repeat step 4.

Expected result:
The website falls back to default language (English)

Actual result:
404 error

Technical details

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