[The Squatter of Amon Rûdh]

I believe it influenced some of his created languages, but I don't know which it would be. Possibly Qenya, or not.

I may well be wrong, but I thought that Quenya was based on old Norwegian.

I once heard a recording of some Polish folk songs, sung by ordinary people who'd been singing them since childhood. JRRT would have loved that sort of thing, as it's the traditional means of passing on knowledge in a pre-literate society (they sounded quite nice too). His own feelings regarding English mythology, not to mention his earlier opinions about the Norman conquest, are widely known.