The Wikimedia Babel Wiki is a proposed central repository for translation/localization information and content related to Wikimedia projects and MediaWiki itself.
Babel would prevent duplication of localization efforts among different projects and users by having a single place to store all varieties and versions of localizations. For instance, one could find new language files for MW 1.5.x even after MW 2.0 comes out; and translation of both the core MediaWiki strings and the customized strings specific to Wikimedia projects.
Babel would also centralize the many multilingual statistics/updates pages scattered across the projects (such as those currently on meta, individual WPs, &c). It would provide would-be project and software users with a snapshot of l10n development in their language (what % of MW strings translated, how many projects being developed in that language, how many users who know that language), and a single place to check for updates (when a string is added to / changed in the software, when a common template is changed in one of the core languages [or in any language]).
Babel would work towards producing on-wiki content that is easily synchronizable with third-party editing/translation/content-management interfaces. Please help develop a project plan in greater detail on the talk page.
Pages to be moved to the new siteEdit
Currently each Wikimedia project language -- every language of Wikipedia, Wiktionary, &c. -- maintains and updates their core custom strings/templates independent of other languages. For instance, when the "there is no article by the name foo" template is changed on de, there is no simple way for localizers into other languages to note this and consider propagating the change to their language.
Moreover, there is no clean way to request or find the latest new strings, or changes to the default strings, in the latest version of MediaWiki; if you are maintaining the Welsh-language interface for Wiktionary, you have to check by hand whether a) your latest suggested changes made it into the release, b) default strings have changed in de/en/es/fr/ja/pl that might prompt changes in welsh, or c) someone else has also added/changed welsh strings since the last release.
On Babel, this work would still be done somewhat 'by hand' -- no new software is proposed for the wiki -- but there would be clear guidelines for how to check for, create, and propagate changes to l10n strings; and a community of translators working on policy and workflow to make this more efficient and transparent.
Each language would have a clear portal for all information relating to the development of content in that language through Wikimedia. Language wikis, translation & interwiki projects, other language portals on individual projects (source, meta, commons, &c), and so forth.