[Answered ]-Wagtail i18n: only show links to pages that are translated

To find pages that are in the locale’s language tree, but not translated there is a alias_of property.

template.html

{% if self.alias_of %}
   not translated
{% else %}
   translated
{% endif %}

views.py

    if self.alias_of:
        is_translated = self.alias_of.locale is self.locale

For more hidden goodies, check the wagtail-localize repo. I found this in the wagtail_hooks.py