The main medieval sources discussed in Kevin Crossley-Holland’s The Penguin Book of Norse Myths were written in Old Norse.
The book, also published as Norse Myths: Tales of Odin, Thor and Loki, retells myths from the medieval Scandinavian tradition. Its principal source materials include the Poetic Edda and Snorri Sturluson’s Prose Edda, both preserved in Old Norse textual traditions.
Old Norse was the language of medieval Scandinavia and developed into several later languages, including Icelandic, Norwegian, Danish, and Swedish. The Icelandic manuscripts are especially important because many mythological poems and prose works survive in that literary tradition.
Old English is a related Germanic language but belongs to the Anglo-Saxon literary tradition represented by Beowulf. Latin was also used by medieval scholars, but it is not the original language of the Eddic poems. Crossley-Holland’s English book is therefore a retelling and translation-based adaptation, not an Old Norse original.