Liland,
Since you're something of a language aficianado (I checked your website) you might be interested in the Hymnals published by the Welsh National Gymanfa Ganu Association. Check them out at our website (www.wngga.org) on the Publications page.
We also have a secular songbook described there called Caneuon y Ddraig (Songs of the Dragon). This one includes an example of penillion singing, a uniquely Welsh art form. The melody is played on an instrument (often, a harp) and the singer sings a countermelody harmonizing with the instrument. The countermelody is often improvised by those who are really good at it.
Many of the old favorites (and widely known songs) from Gwlad Ganu (the Land of Song) are included, as well as current folksongs such as Yma O Hyd (We're still here), sometimes called the unofficial Welsh National Anthem by Dafydd Iwan.
I'd better quit. This is starting to sound like a commercial!!