I could see him go with the ä - he has it in the WHL and also the WJC. Basically there is no difference between "Bärtschi" and "Baertschi", as ä and ae are the same in Germany, Austria and Switzerland. He can also just drop it and go with "Bartschi", as Andreas Nödl and Thomas Pöck did. Then again, the "ö" is far more difficult for non-German speakers than the "ä" ... my guess is he'll use the umlaut. "Bear" is surely a better nickname than "Bart"