From the longer Wikipedia page 
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.
Welsh is its main medium of communication. However, it aims to deliver all public services in Welsh and English.
In 1873, a committee was set up to collect Welsh material and house it at the University College, Aberystwyth. In 1905, the government promised money in its Budget, and the Privy Council appointed a committee to decide on the location of the two institutions. Aberystwyth was selected as the location of the library after a bitter fight with Cardiff, partly because a collection was already available in the College. Sir John Williams, physician and book collector, had also said he would present his collection (in particular, the Peniarth collection of manuscripts) to the Library if it were established in Aberystwyth. He also eventually gave £20,000 to build and establish the library. Cardiff was eventually selected as the location of the National Museum of Wales. The Library and Museum were established by Royal Charter on 19 March 1907.
Designed by architect Sidney Greenslade who won the competition to design the building in 1909, the building at Grogythan, off Penglais Hill, was first occupied in 1916. The central block, or corps de logis, was added by Charles Holden to a modified version of Greenslade's design. In 1996, a large, new storage building was opened, and in recent years many changes have been made to the front part of the building. A new Royal Charter was granted in 2006.
The website, in English and Welsh, is