Last couple of weeks I’ve been working on a new project, based on an ad-hoc i18n solution I had in some of my tools.
Toolserver Intuition is new tool (a “meta-tool”, if you will) that end-users won’t interact much directly, but it is used inside other tools. Both my own tools and tools by other developers.
The translations themselves and the framework around it is open-sourced in Wikimedia SVN: https://svn.wikimedia.org/viewvc/mediawiki/trunk/tools/ToolserverI18N.
More technical details and a short guide on how to implement it (if you’re a Toolserver developer as well) can be found on the Intuition docs.
Update 2013-10-18: Code has moved to GitHub: https://github.com/Krinkle/intuition.