Your Germanic ancestor’s “first name” may have technically been Johann, but he might have had a middle name that was actually his “call name,” or the name he was called. Johann George Trautvetter as given on church record in 1798 may have actually been called George Trautvetter–only using Johann George in church records.
Keep in mind that different areas of Germany may have had different naming traditions and that what may be true in one area may not be true in another.
Research and find out. Don’t assume that what “worked” in one place necessarily worked in another.