You could experiment with the display by placing both his German forenames in the "first name" field, then the Leslie in the preferred name field, and the Ernest in the middle name field. Or the Ludwig Ernst in the "first name" field and the "Leslie Ernest" in the preferred name field.
Another option is to put the Ludwig Ernst in the "first name" field, the Leslie in the preferred name, and mention the change to "Leslie Ernest" in the biography (leaving the middle name blank, as it should probably be for a German).
I greatly fear that, no matter which you decide, it will probably look a tad "clunky" with all the names.
It might also help if you add the Germany tag, as those Wikitreers who are German (or better versed in German naming than am I) would be better able to advise.