...by definition traditional music is not in copyright." This does not apply to the arrangements of specific artists.

E. g., Cecil J. Sharp and Oliver Ditson Company took copyright in 1916 on "One Hundred English Folk Songs," arrangements published in a volume in sheet music form.

An example of copyright claim to music arrangement, but not to lyrics-
Elie Siegmeister, arr., 1964, "The Joan Baez Songbook," Ryerson Music Publishers-
"All of the piano arrangements of public domain songs are Copyright 1964 by Elie Siegmeister and may not be reprinted in any form without permission."
"Joan Baez makes no copyright claim to the authorship or arrangement of any songs in this book."

Much has been posted in Mudcat (threads above linked by Joe Offer), some correct and some not.

Although there is no "international copyright," a number of countries are signatories to the Berne Convention, including the United States, and, I believe, the UK.
See FL 100 (International Copyright) in the U. S. Government list of circulars and factsheets:
http://www.copyright.gov/circs/
U. S. Circulars

The above link is a good place to start in finding U. S. basic Copyright regulations.