When Tim Starling announced the release of MediaWiki version 1.13.0, he mentioned for the first time our localisation efforts: "59,000 new localised text messages have been added, taking the total to 266,000, spread across 281 languages". This is important because it is indicative of the importance that internationalisation and localisation has for the Wikimedia Foundation.
Having 29% more messages localised relative to what was localised at the 1.12.0 release is an impressive number. However, when you look at the "group statistics", you will see that there is a lot more work that needs to be done. There were 172 new core messages since the previous release, Betawiki actually supports 317 languages so on average each language gained 14,2 messages or 0,73%. Obviously some languages did well and others did not.
Tim's announcement is about the MediaWiki core software. Most MediaWiki installations use several of the more then 800 extensions. With the release of MediaWiki the extensions, the 300 that reside in the WMF subversion have also had their code branched. Several of these extensions are under active development and consequently it is not a given that these will function as designed. It is for this reason that an initiative is formed to provide an environment where extensions can be tested in a "plain vanilla" environment.
The Wikimedia Foundation always uses the "bleeding edge" software and consequently the releases provide an environment that is stable for the core. The localisation is ongoing and we can and do release language packs after the release. The extensions are not guaranteed to work on the new release, so we are getting this environment where we can test this and publish the results.
The whole idea is that MediaWiki is easy to use. We want to remove nasty surprises and, we want to create an environment where extensions are supported for the stable releases.