We’ve just released the Language Interface Pack for Windows 7 in Welsh. It’s free as a result of the collaboration between Microsoft and Welsh Language Board. As the press release makes clear, we pay for the work, and the Welsh Language Board provide the translators through the Cymen translation company of Caernarfon. They then get busy translating big swathes of Windows 7 into Welsh with us.
You can read the announcement from the Welsh Language Board for more details.
I’m told that this page contains the download (well, it certainly looks Welsh, doesn’t it)
There's a complete "How to Install with Welsh Language pack" guide here
We have plenty of other things in Welsh too – like Office 2007, Office 2003, Digital Literacy Curriculum and SharePoint ar gael in Gymraeg. And there are 48 other languages for Office here