Ossian

In Werther there is a reference to Ossian, the Gaelic poet.  Our hero, Werther, is supposed to have translated Ossian into German.  The great aria "Pourquoi me Reveiller" is supposed to be Werther reading his translation aloud.

Goethe, the author of the novel Die leiden des jungen Werthers on which the opera is based, was a great admirer of Ossian.  Or James Macpherson depending on which side of the argument you come in on.  The poems were first published in English in 1760 by Macpherson who later published Gaelic versions.  So either the whole thing was a literary hoax, or there is some reason to believe that Macpherson had some Scottish sources.  

Imagine my surprise.  Apparently this argument is still going on.