Move some l10n/i18n information from the wiki

This moves some of the old information from the wiki to the placeholder page. It also redirects for detailed install instructions per platform although they in many cases point back to this page expecting a common, more general, approach to installing the translation files, which applies to all but Wordpress and Drupal 9 which have their own. This needs to be handled somehow (should probably have a TODO).

Note that this page should/could be the landing page for the link on Administer->Languages, Currency, Location unless there is a more user oriented descripition on the options on that page.