lang.Merge

As an example:

{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

A more practical example is to fill in the missing translations from the other languages:

{{ $pages := .Site.RegularPages }}
{{ range .Site.Home.Translations }}
{{ $pages = $pages | lang.Merge .Site.RegularPages }}
{{ end }}