By 1933 Moses was a fluent German speaker, a capability acquired since his boyhood. The Polish tongue, besides being a very complex one, was secondary one for the Iidiche speaking Jews of most of Eastern Europe. Portuguese would be conquered later on, with a great deal of effort and the resulting scars of a strong and ever lasting German accent.
Since the letters are the soul of the movie, it's more than natural that our beloved ghosts speaks to us in German, with its peculiar metaphors and expressions of feelings and state of mind.
Of course the final product will have subtitles, as many as necessary, starting with English and Portuguese, but we expect to make the German gestalt permeate the movie making it at the same time a universal, a Jewish and a German story.