option to not append language code to slug for syncronized trees

[gannebamm]

Issue summary

The translated pages will get appended language codes. These are redundant, since the language identifier is already set behind the root page. Look at the following setup:

/ - ROOT : TranslatablePage as root element
/de : Welcome site in german
/de/blog : Blog listing site in german
/en : Welcome site in english
/en/blog-en : Blog listing in english

The -en is not needed for the blog listing. It will get worse by adding more subpages, since every single one of the will get a language code appended.

How to reproduce?

create a new page in the default language and see how the language code gets appended in the slug.

/ - ROOT : TranslatablePage as root element
/de : Welcome site ingerman
/de/blog : Blog listing site in german
/en : Welcome site in english
/en/blog-en : Blog listing in english

Technical details

• Python, django, wagtail and wagtailtrans version
  python 3.8,
  Django=3.0.6,
  wagtailtrans==2.1