HTML build: Howto append language to internal link URLs

A HTML build of the Mallard pages are published at 
<https://help.ubuntu.com/stable/ubuntu-help/>, similar to 
<https://help.gnome.org/users/gnome-help/stable/>. However, there is one 
important detail which differs: The URLs of the internal links on the 
GNOME site include a language suffix. For instance, while the link at 
the index page to "Your desktop" looks like this on GNOME:

<a href="shell-overview.html.en" title="shell-overview">Your desktop</a>

it looks like this on Ubuntu:

<a href="shell-overview.html" title="Your desktop">Your desktop</a>

i.e. we don't have the ".en" (or ".fr" or ".de" etc.) suffix.

So I have a simple question: How do you do that?

The GNOME model appears to make it easier to navigate the site in some 
other language but the one which is set in the web browser, and I think 
that changing this in Ubuntu would address 
<https://launchpad.net/bugs/1532537> nicely.

Any pointer/guidance on how to add such language suffixes to the 
internal link URLs would be much appreciated.

Thanks!

-- 
Gunnar Hjalmarsson
https://launchpad.net/~gunnarhj