Germany's only joint issue of 2018 was issued in July with South Africa. Like usual the the German Post also wanted to release some joint products, but due to some problems with the South African Post it took two months for them to arrive in Germany. When I bought the new German stamps on Thursday, I also bought the joint First Day Sheet.
This is it:
100th birthday of Nelson Mandela - Germany (issued 12-07-2018)
100th birthday of Nelson Mandela - South Africa (issued 18-07-2018)