en: various fixes

en: various fixes

- update language picker in index, mac and windows;
- add a language picker to workshops;
- use same JScript in workshops as in index, to make lang. picker work;
- remove </p><p> after infographic (to align the paragraph);
- remove spurious <p></p>;
- remove <p> inside <li> (it made the text smaller);
- update <meta> content: 40 instead of 30 min;
- fix a few comments;
- add links to free software definition in mac and windows (reported
  missing by victorhck);
- fix these links to take advantage of MultiViews:
  https://www.gnu.org/philosophy/free-sw.html