Most of the best songs in folk traditions have changed a fair bit since they were made up - just have a look at some broadsheet versions of songs that passed into the oral tradition.

More often than not, the changes have improved the songs. So whether its a translation or not, rewriting has a useful place. The fact that a translation isn't true to the original isn't in itself a reason to put it down, if it works as a song.

But there is a point beyond which it isn't a translation any more, but a new song entirely, with an old tune maybe. (And sometimes tunes need rewriting to fit in with a different culture.)