There can't be the tiniest tour of Langlais's music without some piece based on gregorian chant and some piece based on traditional melodies from Brittany.
Concerning the gregorian inspiration, there are fine exemples in several pieces from the "Hommage à Frescobaldi" recently uploaded by David (McKindl).
Here is one for the Britanny inspiration. It's one the" 8 Chants de Bretagne" composed in 1974. It's late Langlais but, here, he went back to the language of his earlier compositions. It's a nice mix : a modal traditional melody, full of melancholy, supported by lush 20th harmonies.