Q, it's explained at the top of page 247 but it's in French.
To find what is actually in the book in alphabetical order (you need to check after you've removed le, la, l' or les if the title begins by any of these articles) you must go to p. 275.
In the table of themes, in each section, the first titles which are in lower case and italics ARE in the book, then there are titles mentioned in bold capitals with the name of their authors: those are NOT in the book, they're mentioned to let people know they exist.
In Berthier's 1000 chants, you'll find all the foreign songs listed in the table of themes in "Pays, terroirs" in which you first find the names of different countries or groups of countries then the names of different regions of France.

Now a word about "folk song": it doesn't have the same meaning in French (!). A "folk song" (English words) for us is any American traditional song with or without author(s). You might not consider all of them either folk songs or trad songs in your culture. They're usually the type you can listen in westerns or a song by Woody Guthrie and the like. We use "chanson folk" to mean French traditional songs that people such as Malicorne revived in the 70's. Traditional songs from whatever other country is a "chanson traditionnelle".