Really, this whole "train in German" thing is comical to me.
For us there are a few reasons. Back in the early 80's when my husband first got involved in Schutzhund, all the judges were flown over from Germany to judge trials. The judges just preferred if you were giving German commands, so that's what everyone did. That of course became habit.
Also, we don't want the average joe to know what we are telling the dog if it comes down to it. I once had a guy follow me to my car at night (he was a homeless guy looking for change) which made me understandably nervous. The dog that was in the car waiting for me was a young girl just starting in Schutzhund. When I got close enough to the car I told her "Gib Laut" and she totally went nuts since she loved being given permission to bark. The guy hit the high road and of course had no idea all I'd done was tell her to speak.
German commands (the ones we use) are a bit more demonstrative, especially at a distance. It is very hard to make the words down or come heard from a distance we have discovered, whereas platz and hier can be said loudly and still sound right. There are other words that some people use, we don't insist that people in our club use German, some use Czech, some use French, whatever works. But we do tell people to go out on the field and yell the words they are planning to use to make sure they are easy to hear and say. Some people at our club use a combination and that is fine, too.
And finally, we use German for Schutzhund, but will use English around the house for similar commands. For example, Platz means drop immediately and stay there until told otherwise, whereas Lay Down means go find a spot in the house and lay down out of the way. They absolutely get the difference and it goes a long way in not confusing the dog about when they should lie down at attention awaiting the next command or just chilling out.