Adding localization

It's now very simple to translate the web site:
  - first create a new file languages/'your lang'.json based on en.json;
  - add your language at the first lines of dev/generate-static.php;
  - run php generate-static.php in dev/
a new file for your language should be present at the root.

Any modification at the site should be done at dev/index.php

A small server-side program should determine which language choose for the visitor and redirect to the correct file.