Going by the Wikia list, and having listened to some of the works of Percival Schuttenbach, I think I can identify the following songs being based on existing works:
Note that since Percival performs what they call folk metal, it's quite possible that different versions may vary significantly, across album records and live performances, especially since now they seem to perform a mixture of their own songs and the soundtrack songs.
"The Trail", which is also featured in the eponymous opening cinematic, is based both on "Słyszę (1)" and "Sargon" (see below).
"Eyes of the Wolf" is based on (in this case, shares the lyrics) "Oj Dido", from Percival's album of the same name. I wasn't able to find a translation.
"Whispers of Oxenfurt" is based on "Bonny at the Morn" (example performance), and English folk song. To quote the site above,
Northumbria is the only part of England with its own regional music-dialect, its own stock of melodies that are distinct in style from tunes anywhere else in the country. And of this style, Bonny at Morn is one of the masterpieces. Its peculiarity no doubt derives from the character of the local northeastern bagpipe, and the tune was surely an instrumental one before words became attached to it. A great, if neglected, pioneer folk song collector, John Bell, noted the song at the outset of the nineteenth century, but it wasn't printed until 1882, in the Northumbrian Minstrelsy. The poem takes a curious twofold form; in part it's a lullaby addressed to a baby, and in part it's reproach to a lazy son who is 'ower lang' in his bed and won't get up.
"Silver for Monsters..." is based on "Sargon", from the album named Oj Dido (2008).
"... Steel for Humans" is based on "Lazare", from the album Eiforr. "Lazare" is actually a Bulgarian folk song (alternative performance), which is traditionally performed on Lazar's day, 8 days before Easter. Lazarkas - unwed girls - walk from house to house in the village, perform some folk songs, and wish people good harvests and prosperity. I assume Bulgarians were quite amused to hear it while cutting people to sushi in the game.
"Ladies of the Woods" was based on "Saga", also from Eiforr, and available for download 4.6 Mb MP3 from their website.
"You're... Immortal?" (from Hearts of Stone) was based (somewhat loosely, it seems) on "Dziewczyna Swarożyca" (performed live), which was supposedly written before the game, but never recorded until 2014.
"Merchants of Novigrad" was based on "Eiforr" 2.4 Mb MP3 from the eponymous album. It also shares some melodies with "Słyszę (2)".
The rest of the songs I wasn't able to identify myself, so I referred to Internet. The Russian website Pikabu and Quora identified some of the other songs:
"The Song of the Sword-Dancer" is based on a Belarussian folk song "Бегла старожа" (you can listen to a version of it here). To quote Quora,
It is a Belarusian folk carol called Бегла старожа. The phrase repeating after each line literally means Good evening, but rather than a normal greeting, it looks like a reference to Christmas. My interpretation is that on Christmas Eve Khazars (violent Medieval Turkic people who were enemies of Eastern Europeans) attacked some village/city and abducted Sergey’s daughter. The guard wakes everyone and wants to go to war and marry the girl when once he rescues her.
(I don't speak Belarussian, but that seems to be more or less the case based on my knowledge of Russian.)
"The Fields of Ard Skellig" is based on an 18th century Scottish Gaelic song "Fear a' Bhàta" (The Boatman), which has the distinction of having its very own Wikipedia page. Here is an example performance.
"Widow-Maker", by Mikołaj Stroiński, is based on a Croatian folk song named "Naranča" ("Orange", as in the fruit), which is also in Percival's 2012 album Slava! (listen here). According to second-hand Internet sources, it sings of the sea, cold Northern wind, and wine.
I'm in no way sure this list is complete, so criticism and corrections are most welcome.
The band performs some of those songs on multiple occasions. I like this concert in Lublin, and this performance during Ragnarok Rock Fest.
Some of the songs from the soundtrack have been performed by an orchestra with the band during the Film Music Festival in Kraków, and the concert is available for purchase on GOG.