use the model editor. Drag your function object to the source box, and select the triple Fn Language... from the upper central box, and select your corresponing language from the combo box at the upper right box (press enter). Language should be the one associate to your deployment platform (jdbc/odbc/rpg400 or rpgiv...)

By the way, ProcessGroup, differently to other functions, does not comes with a implementation language, because is usualy designed as a pattern or template. It´s preferable to inherit from it, and create a new function appart. If you generate it as part of a general build, but actually not in use, ignore the warning about generation language. It is usual it happens.