i18n is an abbreviation for InternationalisatioN. It's not a specific toolset or framework, more a general term used for anything to do with presenting websites in multiple languages. There's a good article at Wikipedia.
There's also l10n, which is an abbreviation for LocalisatioN. The 10 has to do with the number of letters between the 'L' and the 'N' Localisation goes a little further than just translating websites, as it also has to do with stuff like date, number and currency formats.
i18n-ing or l10n-ing existing websites can be a lot of work, there isn't really a a script you can add to do that. However, if you're starting a new website, it's not so much a problem. Systems like Wordpress have lots of facilities for i18n and l10n, saving you a lot of work. If you want to update your existing website, then the best starting point is probably to look at the gettext suite of tools ([gettext, [URL="http://nl3.php.net/gettext"]php](http://www.gnu.org/s/gettext/)).