Microsoft partnered with the Welsh Language Board to develop first Welsh Language Interface Packs, for Windows XP and Office 2003, and has continued to develop these as new versions came along - with latest being Windows 7.
As Alun said: "Everybody's accepted the new interface easily. The technology that used to be in English is now in Welsh. Staff have taken to it well because it's easy to integrate it into more lessons. Even people who were used to working on an English menu are using it more because it supports the ethos of our school -- providing an immersive education in Welsh."
You can download the full range of Welsh language packs for Windows 7, Vista, XP, Office 2007, Office 2003, SharePoint 2007 and SharePoint Services.
Here's quick links for most recent Language pack in Welsh: