The National Library of Wales (Welsh: Llyfrgell Genedlaethol Cymru), Aberystwyth, is the national legal deposit library of Wales; one of the Welsh Government sponsored bodies. It was founded in 1907.
Welsh is its main medium of communication. However, it aims to deliver all public services in Welsh and English.
More information on the Wikipedia page [1] (English) and [2] (Welsh).
The website, in English and Welsh, is [3].