Board Thread:Support Requests - Designing Your Wiki/@comment-5716479-20130604195804/@comment-10688548-20130604210845

i18n is of course shorthand for the awfully-long word "Internationalization", or preparing things for international and foreign-language environments.

The code is there because it's default code, as can be seen at c:starter:MediaWiki:Wikia.css, from the Starter Pages wiki off of which every single new wiki's default articles are copied. I don't have a blasted idea what it modifies, though