The word Eldar may refer to the ancient people who inhabited the world prior to the birth of man (otherwise known as the Ancient Peoples, the Children of the Gods or the Elder Ones). It may also refer to the language spoken by druids and shaman during rituals, said to be descended from their ancient speech.
Behind the fourth wall...Edit
To represent the Eldar tongue in the game thread, Welsh is used as a stand-in... or pidgin Welsh, anyway. Typically, a sentence is translated using Google Translate. The purpose is not to creat a literal translation, but rather to create the impression of a foreign or alien tongue. Eldar is not meant to resemble a celtic language.