Jean S and Anne Christine Scheeffer had a boy in 1775 named Guillaume Schwingeler in Grosvernick. There is then a marriage record of Guillaume (including the names of his two parents) getting married to Anne Gertrude Weber in Rheinbach. Guillaume is the Frence version and Wilhelm is the German Version. They have nine kids beginning in 1805. The first three have the father's French name on their birth record, while the rest have the German version. This also explain how they ended up in Rheinbach since the marriage certificate states it took place there.
You should use his name at birth. If he later in life wrote (or others wrote) his name differently, you should document this in his About me section. Geni doesn't have the ability yet to let a person have several different names.