The translations are done by native speakers. From time to time I re-check even ones that have been being used for decades by everyone making peace poles. When a language is requested that has not been translated into "May peace prevail on earth" yet, I look for multiple sources. Sometimes it takes longer to work out the translations than it does to make the peace pole.
A couple of times I kept a log of the steps taken to get a translation done. If you would like to read an example so you can see how it is done, check out my page on the Lakota translation.