I believe that if you are German that you will have to deal directly with the LBA and only the LBA.
I am an Irish national but had CAA licences (U.K.), I was connying in DUB FLS and enquired about converting while there. the reply from the IAA was no way.
I had to convert thru the CAA.
I'm afraid I think the South African may have been telling porkies, I have looked up converting a non-jaa licence to a JAR 66 and it does not make pleasant reading.
See JAA Administration & Guidance Material, Section 2 Maintenance, Part Two Procedures, Chapter 40: Conversion of non-jaa licences for info.
As laid back as the Irish are, when it comes to Rules and Regs. the IAA are one of the toughest I have ever dealt with.