- Each top-level webpages is given a category, so as to use a
  different skeleton (article-#categ#.html). The walkthroughs use the
  same layout and hence can all share the same category, and the same
  skeleton.

- forcer_lang is not used; select_lang=no is used instead (I like when
  visitors get all the interface in a foreign language, when
  consulting a foreign walkthrough).

- I planned to use 1 URL and the ?lang= parameter (plus forcer_lang)
  (no 'blah,article#' URLs); it turns out that it would mess up links
  from the editor ([text->#], and forces all skeleton to use some
  loops to workaround the default URL scheme. If we want cleaner URLs,
  better write inc-url-propres3.php...

- I wrote that one: propres-langue.php, based on
  ecrire/urls/propres.php

- Special files to consider when upgrading/backing-up/etc:
  * squelettes/
  * dinksols (symlink to squelettes, backward compat)
  * config/mes_options.php (enable propres-lang.php)
  * ecrire/urls/propres-lang.php (URL rewriting)