All the Witnesses care about is that it is a legal marriage (you have a marriage license registered with the city/county etc) It doesn't matter what priest / rabbi / shaman / imam married you, or what religion it was...as long as it's a legal marriage.
** not sure what they'd do regarding your other ?s.ex: If it's a guy who had multiple wifes etc...in that case you'd probably have to pick a favorite wife, and re do the marriage legally in that specific country.