Page tree problems after translating including subtree

[leewesleyv]

When translating a page through the Wagtail admin (localize/submit/page/<page_id>) and including the subtree, the (translated) page tree is corrupt. The numchild value for the translated page stays 0, which causes the subpages to not appear in the Wagtail admin. I've been able to reproduce this on multiple Wagtail instances.

How to reproduce

On a fresh installation of Wagtail (using wagtail start), configure i18n using the Wagtail docs. Install wagtail-localize following the installation guide. Then perform the following actions:

• Add a new locale
• Add a subpage to the existing start page
• Translate the start page for the newly added locale, including subpages