Sorry, that was not my suggestion.
There is no other key in German for “redo”. It’s the same as in any other language and any other program: ctrl+z=undo, ctrl+y=redo. The use of ctrl+j is an error in the german translation of the help text (perhaps because in other contexts “y” stands for “yes” and in this cases the german translation would be “ja”).
Please just change the “J” to “Y” in the German help text.