Seq("a", "b", "c", "d") => "a, b, c or d"

PrettyStringList.scala

def orJoin(list: Seq[String]) = {
  (list.reverse match {
    case last::secondToLast::rest => (s"$secondToLast or $last"::rest).reverse
    case _ => list
  }).mkString(", ")
}